Change i16x8.splat to use Punpcklqdq instead of Pshufd as the final step
to move low 32 bits to all lanes.

Move this implementation to shared-macro-assembler and use it
everywhere.

Bug: v8:11589,v8:12090
Change-Id: I968b1dca5a262e4e67875caea18c5c09828cb33a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3092558
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76353}