cmd/compile: use generics for isPowerOfTwo predicates

cmd/compile: use generics for isPowerOfTwo predicates

Change-Id: I097b53e9f13de6ff6eb18ae2261842b097f26390
Reviewed-on: https://go-review.googlesource.com/c/go/+/615197
Auto-Submit: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>