nir,radeonsi: Rework XFB handling a bit
The NIR XFB support has grown haphazardly over the years. It started with VK_EXT_transform_feedback and then grew some with @tarceri doing linking in NIR and then a grew a few additional limbs when @mareko plumbed XFB info into lowered store_output
for use in radeonsi. It's time we cleaned it up a bit.
Posting this as a Draft so I can use CI. I'll drop the draft status when I'm ready for review.