cmd/compile: fold negation into comparison operators

cmd/compile: fold negation into comparison operators

This allows for example AMD64 ssa to generate
(SETNE x) instead of (XORLconst [1] SETE).

make.bash trigger count on AMD64:
691 generic.rules:225
  1 generic.rules:226
  4 generic.rules:228
  1 generic.rules:229
  8 generic.rules:231
  6 generic.rules:238
  2 generic.rules:257

Change-Id: I5b9827b2df63c8532675079e5a6026aa47bfd8dc
Reviewed-on: https://go-review.googlesource.com/28232
Run-TryBot: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>