This saves one pointer in Assignment for non-compound
assignment expressions.

Change-Id: I7ec32c1d378917c81ab55c42733b6af450ce65db
Reviewed-on: https://chromium-review.googlesource.com/612673
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47380}