[release-branch.go1.14] cmd/compile: sign extend constant folding properly

MOVLconst must have a properly sign-extended auxint constant.
The bit operations in these rules don't enforce that invariant.

Fixes #42755

Change-Id: I729afcad18752d9b7739e49709020e3be7b3653e
Reviewed-on: https://go-review.googlesource.com/c/go/+/272030


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>