cmd/compile: use isUnsignedPowerOfTwo rather than isPowerOfTwo for unsigneds

cmd/compile: use isUnsignedPowerOfTwo rather than isPowerOfTwo for unsigneds

Fixes #76688

Change-Id: Icb8dab54a5ce7d83b656d50d5ea605d2a62b96f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/726680
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>