cmd/compile: fix load-combining rules

cmd/compile: fix load-combining rules

CL 33632 reorders args of commutative ops in order to make
CSE for commutative ops more robust.  Unfortunately, that
broke the load-combining rules which depend on a certain ordering
of OR ops' arguments.

Introduce some additional rules that order OR ops' arguments
consistently so that the load-combining rules fire.

Note: there's also something else wrong with the s390x rules.
I've filed #19059 for that.

Fixes #18946

Change-Id: I0a5447196bd88a55ccee683c69a57b943a9972e1
Reviewed-on: https://go-review.googlesource.com/36911
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>