cmd/compile: allow multiple SSA block control values

Control values are used to choose which successor of a block is
jumped to. Typically a control value takes the form of a 'flags'
value that represents the result of a comparison. Some
architectures however use a variable in a register as a control
value.

Up until now we have managed with a single control value per block.
However some architectures (e.g. s390x and riscv64) have combined
compare-and-branch instructions that take two variables in registers
as parameters. To generate these instructions we need to support 2
control values per block.

This CL allows up to 2 control values to be used in a block in
order to support the addition of compare-and-branch instructions.
I have implemented s390x compare-and-branch instructions in a
different CL.

Passes toolstash-check -all.

Results of compilebench:

name                      old time/op       new time/op       delta
Template                        208ms ± 1%        209ms ± 1%    ~     (p=0.289 n=20+20)
Unicode                        83.7ms ± 1%       83.3ms ± 3%  -0.49%  (p=0.017 n=18+18)
GoTypes                         748ms ± 1%        748ms ± 0%    ~     (p=0.460 n=20+18)
Compiler                        3.47s ± 1%        3.48s ± 1%    ~     (p=0.070 n=19+18)
SSA                             11.5s ± 1%        11.7s ± 1%  +1.64%  (p=0.000 n=19+18)
Flate                           130ms ± 1%        130ms ± 1%    ~     (p=0.588 n=19+20)
GoParser                        160ms ± 1%        161ms ± 1%    ~     (p=0.211 n=20+20)
Reflect                         465ms ± 1%        467ms ± 1%  +0.42%  (p=0.007 n=20+20)
Tar                             184ms ± 1%        185ms ± 2%    ~     (p=0.087 n=18+20)
XML                             253ms ± 1%        253ms ± 1%    ~     (p=0.377 n=20+18)
LinkCompiler                    769ms ± 2%        774ms ± 2%    ~     (p=0.070 n=19+19)
ExternalLinkCompiler            3.59s ±11%        3.68s ± 6%    ~     (p=0.072 n=20+20)
LinkWithoutDebugCompiler        446ms ± 5%        454ms ± 3%  +1.79%  (p=0.002 n=19+20)
StdCmd                          26.0s ± 2%        26.0s ± 2%    ~     (p=0.799 n=20+20)

name                      old user-time/op  new user-time/op  delta
Template                        238ms ± 5%        240ms ± 5%    ~     (p=0.142 n=20+20)
Unicode                         105ms ±11%        106ms ±10%    ~     (p=0.512 n=20+20)
GoTypes                         876ms ± 2%        873ms ± 4%    ~     (p=0.647 n=20+19)
Compiler                        4.17s ± 2%        4.19s ± 1%    ~     (p=0.093 n=20+18)
SSA                             13.9s ± 1%        14.1s ± 1%  +1.45%  (p=0.000 n=18+18)
Flate                           145ms ±13%        146ms ± 5%    ~     (p=0.851 n=20+18)
GoParser                        185ms ± 5%        188ms ± 7%    ~     (p=0.174 n=20+20)
Reflect                         534ms ± 3%        538ms ± 2%    ~     (p=0.105 n=20+18)
Tar                             215ms ± 4%        211ms ± 9%    ~     (p=0.079 n=19+20)
XML                             295ms ± 6%        295ms ± 5%    ~     (p=0.968 n=20+20)
LinkCompiler                    832ms ± 4%        837ms ± 7%    ~     (p=0.707 n=17+20)
ExternalLinkCompiler            1.58s ± 8%        1.60s ± 4%    ~     (p=0.296 n=20+19)
LinkWithoutDebugCompiler        478ms ±12%        489ms ±10%    ~     (p=0.429 n=20+20)

name                      old object-bytes  new object-bytes  delta
Template                        559kB ± 0%        559kB ± 0%    ~     (all equal)
Unicode                         216kB ± 0%        216kB ± 0%    ~     (all equal)
GoTypes                        2.03MB ± 0%       2.03MB ± 0%    ~     (all equal)
Compiler                       8.07MB ± 0%       8.07MB ± 0%  -0.06%  (p=0.000 n=20+20)
SSA                            27.1MB ± 0%       27.3MB ± 0%  +0.89%  (p=0.000 n=20+20)
Flate                           343kB ± 0%        343kB ± 0%    ~     (all equal)
GoParser                        441kB ± 0%        441kB ± 0%    ~     (all equal)
Reflect                        1.36MB ± 0%       1.36MB ± 0%    ~     (all equal)
Tar                             487kB ± 0%        487kB ± 0%    ~     (all equal)
XML                             632kB ± 0%        632kB ± 0%    ~     (all equal)

name                      old export-bytes  new export-bytes  delta
Template                       18.5kB ± 0%       18.5kB ± 0%    ~     (all equal)
Unicode                        7.92kB ± 0%       7.92kB ± 0%    ~     (all equal)
GoTypes                        35.0kB ± 0%       35.0kB ± 0%    ~     (all equal)
Compiler                        109kB ± 0%        110kB ± 0%  +0.72%  (p=0.000 n=20+20)
SSA                             137kB ± 0%        138kB ± 0%  +0.58%  (p=0.000 n=20+20)
Flate                          4.89kB ± 0%       4.89kB ± 0%    ~     (all equal)
GoParser                       8.49kB ± 0%       8.49kB ± 0%    ~     (all equal)
Reflect                        11.4kB ± 0%       11.4kB ± 0%    ~     (all equal)
Tar                            10.5kB ± 0%       10.5kB ± 0%    ~     (all equal)
XML                            16.7kB ± 0%       16.7kB ± 0%    ~     (all equal)

name                      old text-bytes    new text-bytes    delta
HelloSize                       761kB ± 0%        761kB ± 0%    ~     (all equal)
CmdGoSize                      10.8MB ± 0%       10.8MB ± 0%    ~     (all equal)

name                      old data-bytes    new data-bytes    delta
HelloSize                      10.7kB ± 0%       10.7kB ± 0%    ~     (all equal)
CmdGoSize                       312kB ± 0%        312kB ± 0%    ~     (all equal)

name                      old bss-bytes     new bss-bytes     delta
HelloSize                       122kB ± 0%        122kB ± 0%    ~     (all equal)
CmdGoSize                       146kB ± 0%        146kB ± 0%    ~     (all equal)

name                      old exe-bytes     new exe-bytes     delta
HelloSize                      1.13MB ± 0%       1.13MB ± 0%    ~     (all equal)
CmdGoSize                      15.1MB ± 0%       15.1MB ± 0%    ~     (all equal)

Change-Id: I3cc2f9829a109543d9a68be4a21775d2d3e9801f
Reviewed-on: https://go-review.googlesource.com/c/go/+/196557
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/rewrite.go

@@ -25,9 +25,10 @@ func applyRewrite(f *Func, rb blockRewriter, rv valueRewriter) {
 	for {
 		change := false
 		for _, b := range f.Blocks {
-			if b.Control != nil && b.Control.Op == OpCopy {
-				for b.Control.Op == OpCopy {
-					b.SetControl(b.Control.Args[0])
+			for i, c := range b.ControlValues() {
+				for c.Op == OpCopy {
+					c = c.Args[0]
+					b.ReplaceControl(i, c)
 				}
 			}
 			if rb(b) {