ShenandoahHeap::is_in should check for alive regions

The expected behavior of `CollectedHeap::is_in` is to check whether the object belongs to the committed parts of the heap. This is useful to check if object resides in the parts of the heap the GC knows are not dead. Yet, Shenandoah's check just verifies that oop is within the heap bounds. So `is_in` check for an object that is in trashed/empty region would pass by accident, and we will miss detecting bugs. This should be rectified.

Among other things, the fix would enable JDK-8337978 checks to work for JDK-8337941 failure detection.