The code already checks wether decoder.failed(), so checking whether
AnalyzeLoopAssignment returned null is redundant.

Change-Id: Ia6e8b22011b28c76974aca8a6119466fe343079e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2492329Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70728}