Shenandoah: Invoke native-LRB only on non-strong refs

The way that current native LRB is implemented is wrong (but non-fatal) and misleading. It's purpose is to prevent resurrection of unreachable non-strong references, and it should only be invoked on non-strong references, not all native references. This distinction will become even more important once we get concurrent reference processing: then we also want to invoke this barrier on referent-loads.