The maybe-assigned flag of the catch variable was not preserved.

BUG=v8:5636,chromium:696332

Change-Id: I9c55e1b1312bdebc53bc45bc3ca1c982bdbe9846
Reviewed-on: https://chromium-review.googlesource.com/447680Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43506}