LoadMaybeWeakField returns a TNode<MaybeObject>, and the only way to extract
values from it is through explicit functions.

BUG=v8:7308

Change-Id: I98da6b715527bb9976b4c78ed9caaa39fbcb3ed5
Reviewed-on: https://chromium-review.googlesource.com/1014106
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52732}