Inner pointer resolution, to be used in conservative stack scanning,
assumes that all pages registered with the memory allocator are
iterable. Until this CL, this was not the case for pages that were
owned by the young generation semispaces but were unused. Such pages
are either in the "from" semispace, or in the "to" semispace but have
not yet been used.

This CL ensures that all pages owned by the young generation are iterable. It also adds tests to verify that inner pointer resolution
works correctly for unused young pages and for pointers above the
page area.

Bug: v8:13257
Change-Id: Ieff7cc216853403e01f83220b96bf8ff4cdea596
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3885893Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83226}