cmd/compile: experimental loop iterator capture semantics change

Adds:
GOEXPERIMENT=loopvar (expected way of invoking)
-d=loopvar={-1,0,1,2,11,12} (for per-package control and/or logging)
-d=loopvarhash=... (for hash debugging)

loopvar=11,12 are for testing, benchmarking, and debugging.

If enabled,for loops of the form `for x,y := range thing`, if x and/or
y are addressed or captured by a closure, are transformed by renaming
x/y to a temporary and prepending an assignment to the body of the
loop x := tmp_x.  This changes the loop semantics by making each
iteration's instance of x be distinct from the others (currently they
are all aliased, and when this matters, it is almost always a bug).

3-range with captured iteration variables are also transformed,
though it is a more complex transformation.

"Optimized" to do a simpler transformation for
3-clause for where the increment is empty.

(Prior optimization of address-taking under Return disabled, because
it was incorrect; returns can have loops for children.  Restored in
a later CL.)

Includes support for -d=loopvarhash=<binary string> intended for use
with hash search and GOCOMPILEDEBUG=loopvarhash=<binary string>
(use `gossahash -e loopvarhash command-that-fails`).

Minor feature upgrades to hash-triggered features; clients can specify
that file-position hashes use only the most-inline position, and/or that
they use only the basenames of source files (not the full directory path).
Most-inlined is the right choice for debugging loop-iteration change
once the semantics are linked to the package across inlining; basename-only
makes it tractable to write tests (which, otherwise, depend on the full
pathname of the source file and thus vary).

Updates #57969.

Change-Id: I180a51a3f8d4173f6210c861f10de23de8a1b1db
Reviewed-on: https://go-review.googlesource.com/c/go/+/411904
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

test/escape.go

@@ -126,7 +126,8 @@ func range_escapes2(x, y int) (*int, *int) {
 	var p [2]*int
 	a[0] = x
 	a[1] = y
-	for k, v := range a {
+	var k, v int
+	for k, v = range a {
 		p[k] = &v
 	}
 	return p[0], p[1]
@@ -136,7 +137,8 @@ func range_escapes2(x, y int) (*int, *int) {
 func for_escapes2(x int, y int) (*int, *int) {
 	var p [2]*int
 	n := 0
-	for i := x; n < 2; i = y {
+	i := x
+	for ; n < 2; i = y {
 		p[n] = &i
 		n++
 	}