Covers a bunch of constant-folding rules in generic.rules that aren't
being covered currently.

Increases coverage in generic.rules from 65% to 72%.

Change-Id: I7bf58809faf22e97070183b42e6dd7d3f35bf5f9
Reviewed-on: https://go-review.googlesource.com/23407
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Todd Neal <todd@tneal.org>