Implements v32x4.load_splat and v64x2.load_splat.

Bug: v8:9886
Change-Id: I18f3b012f9980d258985edf2ff26577fe495eff5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1903747Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64866}