[release-branch.go1.18] cmd/compile: disable rewrite loop detector for deadcode-only changes

We're guaranteed we won't infinite loop on deadcode-only changes,
because each change converts valid -> invalid, and there are only a
finite number of valid values.

The loops this test is looking for are those generated by rule
applications, so it isn't useful to check for loops when rules aren't
involved.

Fixes #52366

Change-Id: Idf1abeab9d47baafddc3a1197d5064faaf07ef78
Reviewed-on: https://go-review.googlesource.com/c/go/+/392760
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
(cherry picked from commit 15728ce950)
Reviewed-on: https://go-review.googlesource.com/c/go/+/400514
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -40,6 +40,7 @@ func applyRewrite(f *Func, rb blockRewriter, rv valueRewriter, deadcode deadValu
 	var states map[string]bool
 	for {
 		change := false
+		deadChange := false
 		for _, b := range f.Blocks {
 			var b0 *Block
 			if debug > 1 {
@@ -73,7 +74,7 @@ func applyRewrite(f *Func, rb blockRewriter, rv valueRewriter, deadcode deadValu
 						// Not quite a deadcode pass, because it does not handle cycles.
 						// But it should help Uses==1 rules to fire.
 						v.reset(OpInvalid)
-						change = true
+						deadChange = true
 					}
 					// No point rewriting values which aren't used.
 					continue
@@ -145,15 +146,16 @@ func applyRewrite(f *Func, rb blockRewriter, rv valueRewriter, deadcode deadValu
 				}
 			}
 		}
-		if !change {
+		if !change && !deadChange {
 			break
 		}
 		iters++
-		if iters > 1000 || debug >= 2 {
+		if (iters > 1000 || debug >= 2) && change {
 			// We've done a suspiciously large number of rewrites (or we're in debug mode).
 			// As of Sep 2021, 90% of rewrites complete in 4 iterations or fewer
 			// and the maximum value encountered during make.bash is 12.
 			// Start checking for cycles. (This is too expensive to do routinely.)
+			// Note: we avoid this path for deadChange-only iterations, to fix #51639.
 			if states == nil {
 				states = make(map[string]bool)
 			}