It removes special handling for Cells and PropertyCells. That handling
was required before when new space objects were embedded in code objects
via Cells. Since code objects support direct embedding now, the handling
can be removed.

The patch also makes sure to load the map of the object once using
the synchronized accessor, which will be needed for concurrent visiting
of code object.

Bug: v8:8459
Change-Id: I83833e19ad1da4a92e1a9be60b7c1dcd05c2b2be
Reviewed-on: https://chromium-review.googlesource.com/c/1337745Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57616}