Implement a super sketched out pattern match/rewrite framework and a sketched

out canonicalization pass to drive it, and a simple (x-x) === 0 pattern match
as a test case.

There is a tremendous number of improvements that need to land, and the
matcher/rewriter and patterns will be split out of this file, but this is a
starting point.

PiperOrigin-RevId: 216788604