Most of the Map's fields are being read directly from the heap so
there's no need to serialize them with concurrent inlining on.

As an extra step, read NextFreePropertyIndex directly.

Bug: v8:7790
Change-Id: Ia0c46a3bbb673f9fdea9b4700908f00a43c28e28
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2892665
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74628}