Small int nodes are stored in sign-extended form, for unsigned average,
mask away the top bits before performing operation.

Bug: v8:10507
Change-Id: I04d3be5758e6ee3fd946adca0943b2874910b4cf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2405751
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69892}