cmd/compile: simplify 386's rules

This CL combines several rules together via regular expression,
but won't impact generated 386 code.

Change-Id: I354006fe801fc952e3a9431cae63229922c9ba48
Reviewed-on: https://go-review.googlesource.com/c/139957
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>