Add an early exit if the control op is Dead to prevent failing the
DCHECK.

BUG=chromium:797596, v8:5940, v8:3018
R=bmeurer@chromium.org, jarin@chromium.org

Change-Id: I6090380ea69c3205740b6c7a41d7c066d18d6a9f
Reviewed-on: https://chromium-review.googlesource.com/844978
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50312}