cmd/compile: use unsigned power-of-two detector for unsigned mod

cmd/compile: use unsigned power-of-two detector for unsigned mod

Same as CL 689815, but for modulus instead of division.

Updates #74485

Change-Id: I73000231c886a987a1093669ff207fd9117a8160
Reviewed-on: https://go-review.googlesource.com/c/go/+/689895
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>