This implements support for i31.get_s and i31.get_u.

Bug: v8:7748
Change-Id: Icbfddbc2ff46b4eb6bf3edf7b3a794f9797361d4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2595309
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71808}