If we SmiTag using the same register we will save a mov instruction.

Bug: v8:9771
Change-Id: Iadfa332e76115688eac6c3ec5262ca751ec02405
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1826735Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64029}