cmd/compile: rewrite Rsh to RshU if arguments are proved positive

cmd/compile: rewrite Rsh to RshU if arguments are proved positive

Fixes #76332

Change-Id: I9044025d5dc599531c7f88ed2870bcf3d8b0acbd
Reviewed-on: https://go-review.googlesource.com/c/go/+/721206
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>