WIP: nir: Take out the trash
This MR deletes some unneeded stuff from
nir_register. I've marked it WIP because I'm not 100% sure about some of it (getting rid of uses, in particular) and may want to add more stuff to it as I continue my sprint cleaning. Other NIR maintainers should feel free to tack stuff on to the MR if they find other cleaning and we can land it at some point.