`tkw.debug_log` should check that `register_` is `tkl.Register`

If passed `register_=`, `tkw.debug_log` will eventually fail with an inscrutable error like

```
AssertionError: Vector shapes not set for node write: write(register_=kv_indices, memory=block_table, elements_per_thread=1, mapping_dynamic_vals=(), index={K2: 0 : 1 : 1}) type(Memory[K2].of(i32))
```

`tkw.debug_log` should probably check that `register_` is indeed `tkl.Register` and immediately fail with an informative error message if that's not the case.