I missed actually handling these instructions in liftoff-compiler, so
even though the assembler functions were implemented for all archs, we
weren't running them.

This properly handles the instructions and a couple of fixes:

- for arm64, typos in using signed instructions for unsigned Wasm ops
- for arm, handle the case where dst == src, which leads to us
overwriting src and then reading junk from the overwritten portions to
convert

Bug: v8:11265
Change-Id: I7919280bdf395137e95075deb30ed815100df222
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2728382Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73178}