GitLab

Some optimizations of things I've seen looking at generated code.
  (x+y)-x == y
  x-0 == x
The ptr portion of the constant string "" can be nil.

Also update TODO with recent changes.

Change-Id: I02c41ca2f9e9e178bf889058d3e083b446672dbe
Reviewed-on: https://go-review.googlesource.com/16771


Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>