* Simplify comparisons of form a + const1 == const2 or a + const1 != const2.
* Canonicalize Eq, Neq, Add, Sub to have a constant as first argument.
Needed for the above new rules and helps constant folding.

Change-Id: I8078702a5daa706da57106073a3e9f640a67f486
Reviewed-on: https://go-review.googlesource.com/19192Reviewed-by: Keith Randall <khr@golang.org>