cmd/compile: don't elide zero extension on top of signed values

v = ... compute some value, which zeros top 32 bits ...
w = zero-extend v

We want to remove the zero-extension operation, as it doesn't do anything.
But if v is typed as a signed value, and it gets spilled/restored, it
might be re-sign-extended upon restore. So the zero-extend isn't actually
a NOP when there might be calls or other reasons to spill in between v and w.

Fixes #68227

Change-Id: I3b30b8e56c7d70deac1fb09d2becc7395acbadf8
Reviewed-on: https://go-review.googlesource.com/c/go/+/595675
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Joedian Reid <joedian@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

test/fixedbugs/issue68227.go

@@ -0,0 +1,43 @@
+// run
+
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"fmt"
+)
+
+type someType []uint64
+
+func (s *someType) push(v uint64) {
+	*s = append(*s, v)
+}
+
+func (s *someType) problematicFn(x1Lo, x1Hi, x2Lo, x2Hi uint64) {
+	r1 := int32(int16(x1Lo>>0)) * int32(int16(x2Lo>>0))
+	g()
+	r3 := int32(int16(x1Lo>>32)) * int32(int16(x2Lo>>32))
+	r4 := int32(int16(x1Lo>>48)) * int32(int16(x2Lo>>48))
+	r5 := int32(int16(x1Hi>>0)) * int32(int16(x2Hi>>0))
+	r7 := int32(int16(x1Hi>>32)) * int32(int16(x2Hi>>32))
+	r8 := int32(int16(x1Hi>>48)) * int32(int16(x2Hi>>48))
+	s.push(uint64(uint32(r1)) | (uint64(uint32(r3+r4)) << 32))
+	s.push(uint64(uint32(r5)) | (uint64(uint32(r7+r8)) << 32))
+}
+
+//go:noinline
+func g() {
+}
+
+func main() {
+	s := &someType{}
+	s.problematicFn(0x1000100010001, 0x1000100010001, 0xffffffffffffffff, 0xffffffffffffffff)
+	for i := 0; i < 2; i++ {
+		if got, want := (*s)[i], uint64(0xfffffffeffffffff); got != want {
+			fmt.Printf("s[%d]=%x, want %x\n", i, got, want)
+		}
+	}
+}