Incorrect class pointer check in concurrent marker

[wenyuzhao]

This class pointer check is incorrect:

mmtk-core/src/plan/lxr/cm.rs

Line 109 in df7cd4d

debug_assert!(object.class_is_valid::<VM>());

Concurrent marker may visit a dead object which is already killed by RC decrements. RC dec processing will mark this object right before it is killed to ensure the concurrent marker will never scan these objects. Allocators can reuse this memory at anytime after the object is killed.

At the time concurrent marker visits these objects, they should be ignored since they're already marked.

Since their memory can be reused, their class pointers can be invalid. So the class pointer checks should never apply to these objects.