* Add support for one case of `UOps.CAST` for RDNA3 assembler
* Adds support for casting from `bool` -> `float32`. Seems like a very common operation that is required in many places.
* Fix bool register definition for vector operations
* Use `vcc_lo` instead of `vcc` which seems to be required since it's configured to use wavefront_size=32
* Add vector support for some places that were scalar only in register definition and comparison ops
* Fix some issues in what seems to be defunct `external_test_image.py`
* Some tests still don't pass for other reasons, but it at least runs now and one broken test is now fixed
* Refactor RDNA3 assembler register definition
* Unify multi-registor code between dtypes and combine with single-register allocation since they're all untyped registers at the end of the day
