cmd/compile: add limits bruteforce tests for bitlen

cmd/compile: add limits bruteforce tests for bitlen

This ensure BitLen is perfect within the limitations limits can represent.

Change-Id: I5c1770b4a9f6408fd68fe77b4ef2b2cdd52e26cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/727781
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>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>