cmd/compile: remove isUintXPowerOfTwo functions

cmd/compile: remove isUintXPowerOfTwo functions

And use the generic version instead.

While at it, also correct the corresponding rules to use logXu variants
instead of logXu, following discussion in CL 689815.

Change-Id: Iba85d14ff0e26d45a126764e7bd5702586358d23
Reviewed-on: https://go-review.googlesource.com/c/go/+/692917
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>