x*2 -> x+x
x/c, c power of 2 -> x*(1/c)

Fixes #19827

Change-Id: I74c9f0b5b49b2ed26c0990314c7d1d5f9631b6f1
Reviewed-on: https://go-review.googlesource.com/39295
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>