copy_to_survivor_space should use in-hand klass for scanning

Scanning the oops in an instance is done via obj->oop_iterate_backward(closure). That function gets the klass of obj and calls another function with the obj, closure, and the just obtained class. But copy_to_survivor_space has already obtained the klass for other reasons, and pass it to that call, avoiding refetching it (including dealing with UseCompressedClassPointers).