Drive-by: Fix bug where 8x16ExtractLane could extract garbage value.

Change-Id: I7eaa758c0c649e4e79e64039d1db6e19092d1293
Bug: v8:6666
Reviewed-on: https://chromium-review.googlesource.com/c/1271056
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56493}