Cleanup LoadU64 and LoadU64WithUpdate

Change-Id: If98c6949aeaa9c2b9bca47958cb72d80d70e7309
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2930360
Commit-Queue: Junliang Yan <junyan@redhat.com>
Reviewed-by: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/master@{#74887}