RewritableExpression always contain Assignment until rewriting; which
we only do after parsing a function. We don't call ->Is on possibly
rewritten expressions afterwards (we'll simply visit them instead).

Change-Id: I3be91e9f6701994a571d280515c9da34257f5425
Reviewed-on: https://chromium-review.googlesource.com/c/1291071Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56821}