radv: Add new linking step and use nir_opt_varyings.
Based on:
- !28685 (merged)
- !28704 (merged)
- !28764 (merged)
- !28765 (merged)
- !28768 (merged)
- !29208 (merged)
- !29209 (merged)
- !29210 (merged)
- !29406 (merged)
- !29435 (merged)
- !29895 (merged)
Add a new linking step to RADV which will run after nir_lower_io and will work on lowered I/O. Then, use nir_opt_varyings in this new linking step and gradually migrate the old linking code there.
Edited by Timur Kristóf