containing smi value and untags it.

This CL finishes up the parity with x64 with regards to
(https://chromium-review.googlesource.com/c/v8/v8/+/1382740)

Bug: v8:7703
Change-Id: I3c88fbbfd3e47e944a6891171d6555f330cd5fd2
Reviewed-on: https://chromium-review.googlesource.com/c/1463523Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59521}