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>