Add rewrite rules to optimize constant shifts.

Fixes #10637

Change-Id: I74b724d3e81aeb7098c696d02c050f7fdfd5b523
Reviewed-on: https://go-review.googlesource.com/19106Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>