Adds support to the register allocator verifier to keep track of which
stack slots contain tagged pointers, but have not been tracked by the
reference map and so could contain stale values (i.e., not traced by a
garbage collection).

BUG=v8:9684

Change-Id: I8dd9925f0cb71cac4ae3e49f467767454694e515
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2007488Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65883}