cmd/compile: prefer rematerializeable arg0 for HMUL

This prevents accidental regalloc regressions
that otherwise can occur from unrelated changes.

Change-Id: Iea356fb1a24766361fce13748dc1b46e57b21cea
Reviewed-on: https://go-review.googlesource.com/129375


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>