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: Ibf157382fb4544c063fbc80406fb9302430728fe
Reviewed-on: https://go-review.googlesource.com/95595Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>