cmd/compile: improve Ctz's limits modeling and add bruteforce tests

cmd/compile: improve Ctz's limits modeling and add bruteforce tests

This make Ctz perfect within the limitations limits can represent.

Change-Id: I1e596d8d01892d1b70031cf03cecc487ce147b38
Reviewed-on: https://go-review.googlesource.com/c/go/+/727780
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>