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

cmd/compile: use | in the most repetitive arm 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: Ib1d2b9fbc787379105ec9baf10d2c1e2ff3c4c5c
Reviewed-on: https://go-review.googlesource.com/95615
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>