cmd/compile: use right shifts to eliminate bounds checks

cmd/compile: use right shifts to eliminate bounds checks

These rules trigger a few times during make.bash.
When we eliminate boundedness checks from walk.go
we'll rely on them more heavily.

Updates #19692

Change-Id: I268c36ae2f1401c68dd685b15f2d30f5d6971176
Reviewed-on: https://go-review.googlesource.com/43775
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>