To keep things simple, we just ensure that the spill slots always
contain full uncompressed pointers before GC sees them.

Bug: v8:8977, v8:7703
Change-Id: I54eab1b3e79e8525200139e487ff64d82ae157e5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1564198Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60811}