The AssignmentExpressions can legally contain destructuring assignments.

BUG=v8:6098
R=marja@chromium.org, adamk@chromium.org

Change-Id: I99b3a0f4c8d103edfb1dda943ec3e2ab2a5969f7
Reviewed-on: https://chromium-review.googlesource.com/455221
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44049}