Combining the load with the SmiUntag let's us have better
instruction selection opportunities against not doing it.

Bug: v8:9771
Change-Id: I6b6391398413ee1bc808c71d48474e9336c09f2b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1826734
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64025}