cmd/compile: use ISEL, cleanup use of zero & extensions

Abandoned earlier efforts to expose zero register,
but left it in numbering to decrease squirrelyness of
register allocator.

ISELrelOp used in code generation of bool := x relOp y.
Some patterns added to better elide zero case and
some sign extension.

Updates: #17109

Change-Id: Ida7839f0023ca8f0ffddc0545f0ac269e65b05d9
Reviewed-on: https://go-review.googlesource.com/29380
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -32,8 +32,9 @@ type Config struct {
 	noDuffDevice    bool                       // Don't use Duff's device
 	nacl            bool                       // GOOS=nacl
 	use387          bool                       // GO386=387
+	OldArch         bool                       // True for older versions of architecture, e.g. true for PPC64BE, false for PPC64LE
 	NeedsFpScratch  bool                       // No direct move between GP and FP register sets
-	DebugTest       bool                       // as a debugging aid for binary search using GOSSAHASH, make buggy new code conditional on this
+	DebugTest       bool                       // default true unless $GOSSAHASH != ""; as a debugging aid, make new code conditional on this and use GOSSAHASH to binary search for failing cases
 	sparsePhiCutoff uint64                     // Sparse phi location algorithm used above this #blocks*#variables score
 	curFunc         *Func
 
@@ -180,7 +181,10 @@ func NewConfig(arch string, fe Frontend, ctxt *obj.Link, optimize bool) *Config
 		c.FPReg = framepointerRegARM64
 		c.hasGReg = true
 		c.noDuffDevice = obj.GOOS == "darwin" // darwin linker cannot handle BR26 reloc with non-zero addend
-	case "ppc64le", "ppc64":
+	case "ppc64":
+		c.OldArch = true
+		fallthrough
+	case "ppc64le":
 		c.IntSize = 8
 		c.PtrSize = 8
 		c.lowerBlock = rewriteBlockPPC64