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

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

This make PopCount perfect within the limitations limits can represent.

Change-Id: Ia52e5d79064ad8109117f009c5390a6eba8ccd98
Reviewed-on: https://go-review.googlesource.com/c/go/+/727782
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>