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

Previous version: https://chromium-review.googlesource.com/1014106

BUG=v8:7308

Change-Id: I73bf3a007733ed84937e45336ac142011adc5151
Reviewed-on: https://chromium-review.googlesource.com/1023935Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52750}