The current version of SmiTag and SmiUntag was checking if the
registers were the same, copying them if not and then untagging.
We can avoid a branch and a check by having two versions of
SmiTag and SmiUntag.

Change-Id: Id89213e073cefc9f8e46fcf0e79d0c1d349342ae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1826730Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64021}