This is to fix the pointer compression bot, which tests only x64.
Other 64-bit architectures will need similar work in the future.

On x64 with pointer compression, smi-untagging can be folded into the
Operand calculation since the left-shift for multiplying by
kSystemPointerSize is greater than the right-shift for untagging.

Bug: v8:7777
Change-Id: I5c46e9d3f51580341cfc3c12a7e32d17cf0b63ee
Reviewed-on: https://chromium-review.googlesource.com/c/1381452
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58328}