cmd/compile: add tests bruteforcing limit negation and improve limit addition

cmd/compile: add tests bruteforcing limit negation and improve limit addition

I had to improve addition to make the tests pass.

Change-Id: I4daba2ee0f24a0dbc3929bf9afadd2116e16efae
Reviewed-on: https://go-review.googlesource.com/c/go/+/724600
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>