The compiler is only interested in the contents if it contains a
FeedbackVector. If one is discovered, it is serialized, and we
ensure we'll either return it or nothing if the contents of
the cell changed on the main thread.

FeedbackCells can be reset if the bytecode for the associated
function is flushed. We have guarantees only for functions we
choose to inline that this doesn't happen (by holding a strong
handle to the SharedFunctionInfo).

Bug: v8:7790
Change-Id: I9ecff3f4aef39169d84501feae9e47f2d118054e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2434324
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72260}