I missed out the i8x16 implementation in https://crrev.com/c/2169017.

Bug: v8:9909
Change-Id: I3264e9dce51acca262ad71885379b320008555b5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2173657Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67507}