cmd/compile: don't generate commutative rules for (Op x x)

If the two commutative arguments are perfectly identical,
then swapping them will never have an effect.

Passes toolstash-check for the relevant architectures,
that is, linux-386, linux-386-387, linux-amd64, linux-s390x.

Change-Id: I19f91644867d8d174bd01f872abe4809013872ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/213698


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>