Due to collections with inlined storage, Oilpan still supports on-stack
Members, which are always compressed if pointer compression is enabled.
This CL scans halfwords (together with full words) on stack to find
potential pointers. Since on-heap pointers can only be compressed and
in-construction objects always reside on heap, only halfwords need to be
scanned for them.

The alternative potential followup approaches:
1) Use a separate uncompressed type for pointer in inlined collections;
2) Dynamically register regions of stack containing compressed pointers.

Bug: chromium:1325007
Change-Id: Ia706fd8e7383d30aff11f4014faa9edd3d289a55
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3644959Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80747}