Only use the "W" part (lower 32 bit) of the src register. Otherwise, we
can get results larger than 32.

R=ahaas@chromium.org
CC=​rodolph.perfetta@arm.com

Bug: v8:7914, chromium:854011
Change-Id: I6329231e6cc0ae537c165b2d383fc5a14bd28ca3
Reviewed-on: https://chromium-review.googlesource.com/1122409
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54152}