LoadMem already does the proper sign extension based on the
passed memtype, so all is left is splatting it to the desired
number of lanes.

Change-Id: I54317b6e2b4d3daf2e980315198577f61c3f556a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2068031Reviewed-by: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66436}