Avoid visiting dead large objects in RegionSpace::Walk

The motivation is to prevent large objects from being visited by
RegionSpace::Walk if it is called before the next GC's SetFromSpace
marks the large object as from-space. This fixes possible dangling
pointer issues.

A follow up CL will clear the empty unevac regions.

Bug: 35800768
Bug: 12687968

Test: test-art-host

Change-Id: I6323959f0b7b2a357e6d6483cd1c33fb63c3d54a