- Use movl which clears the upper 32bits on x64
- Use xorl + movb for Smi.ptr values <= 0xFF, saving one byte over movl

Change-Id: Iacdacfbe397670667e71d1d12ef427a01994481d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2642250
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72282}