[release-branch.go1.21] Revert "cmd/compile: omit redundant sign/unsign extension...

[release-branch.go1.21] Revert "cmd/compile: omit redundant sign/unsign extension on arm64"

This reverts CL 427454.

Reason for revert: causes incorrect generated code in some rare cases

We'll fix-forward at tip, so the revert just needs to be done for 1.21.

Fixes #62143

Change-Id: Id242230481ff4d4ba5f58236c6d8237729fc3b80
Reviewed-on: https://go-review.googlesource.com/c/go/+/520976
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Ruinan Sun <Ruinan.Sun@arm.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>