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
TBR=yangguo@chromium.org

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I16d65dc768f10ed431252e23a0df07bee9063534
Reviewed-on: https://chromium-review.googlesource.com/948493
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51731}