This patch simplifies and reduces code duplication & dead branches
in PatternRewriter, especially relating to handling of destructuring
assignments. It also adds additional DCHECKs to capture invariants.

It should have no change in behavior.

Change-Id: I02bc6c30a1cae8a60c4c5e9033b90953d136212e
Reviewed-on: https://chromium-review.googlesource.com/612511
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47382}