Test all the weird shifts, like int8 shifted right by uint16.
Increases coverage for shift lowerings in AMD64.rules.

Change-Id: I066fe6ad6bfc05253a8d6a2ee17ff244d3a7652e
Reviewed-on: https://go-review.googlesource.com/23585
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Todd Neal <todd@tneal.org>