Excluded regions are no longer available to the RegionAllocator, so
should not be freeable so actually enforce that and aAdd a test.

Bug: v8:10454
Change-Id: I51c41cf0bf3d2eeb699b10b1fa02f5465d93b6aa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2330026Reviewed-by: Igor Sheludko (OOO Aug 3-17) <ishell@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69163}