Implement in-place weak reference handling in GC.

Turn FeedbackVector::optimized_code_or_smi into an in-place weak reference (this
is the only in-place weak reference at this point).

(See bug for design doc.)

BUG=v8:7308

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng;master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I0f9f992cb4ee0457c40b7c868317dfb607bfb906
Reviewed-on: https://chromium-review.googlesource.com/873638
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51722}