cmd/compile: use | in the most repetitive mips rules

cmd/compile: use | in the most repetitive mips rules

For now, limited to the most repetitive rules that are also short and
simple, so that we can have a substantial conciseness win without
compromising rules readability.

Ran rulegen, no change in the actual compiler code (as expected).

Change-Id: Ib0bfbbc181fcec095fb78ac752addd1eee0c3575
Reviewed-on: https://go-review.googlesource.com/95796
Reviewed-by: Giovanni Bajo <rasky@develer.com>