cmd/compile: use | in the most repetitive mips64 rules

cmd/compile: use | in the most repetitive mips64 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: Ia74acc389cd8310eb7fe8f927171fa3d292d2a86
Reviewed-on: https://go-review.googlesource.com/95797
Reviewed-by: Giovanni Bajo <rasky@develer.com>