Mostly mirrors ARM, includes:
- constant folding
- simplification of load, store, extension, and arithmetics
- nilcheck removal

Change-Id: Iffaa5fcdce100fe327429ecab316cb395e543469
Reviewed-on: https://go-review.googlesource.com/26710
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>