Move the entry-point for destructuring assignment out of the recursion so we
can avoid swapping ASSIGNMENT scope to ASSIGNMENT_ELEMENT.

Also rewrite Assignment directly without wrapping in RewritableExpression
first.

Change-Id: Iae768ad1b2a6fb40ce37142867d7034f924354e4
Reviewed-on: https://chromium-review.googlesource.com/c/1264284Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56406}