[release-branch.go1.25] cmd/compile: during regalloc, fixedreg values are always available

It is ok to clobber registers that have a copy of a fixedreg value,
as that value is always available in its original location later
if we need it. (See 14 lines below the change.)

This CL will fix the regalloc infinite loop that CL 678620 introduced.

That CL requests that the stack pointer value be materialized in a
non-stack-pointer register, which is atypical. That condition
triggered the infinite loop that this CL fixes.  The infinite loop is
the compiler trying to reuse that non-stack-pointer register for
something else, but then refusing to give it up because it thought
that non-stack-pointer register held the last copy of the original SP
value.

Fixes #75844

Change-Id: Id604d0937fb9d3753ee273bf1917753d3ef2d5d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/696035
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
(cherry picked from commit 9bbea0f)
Reviewed-on: https://go-review.googlesource.com/c/go/+/710875
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>

Author: randall77

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

@@ -2433,7 +2433,7 @@ func (e *edgeState) processDest(loc Location, vid ID, splice **Value, pos src.XP
 	}
 
 	// Check if we're allowed to clobber the destination location.
-	if len(e.cache[occupant.vid]) == 1 && !e.s.values[occupant.vid].rematerializeable {
+	if len(e.cache[occupant.vid]) == 1 && !e.s.values[occupant.vid].rematerializeable && !opcodeTable[e.s.orig[occupant.vid].Op].fixedReg {
 		// We can't overwrite the last copy
 		// of a value that needs to survive.
 		return false