radv: Use NIR barycentric intrinsics

radv: Use NIR barycentric intrinsics

We have to add a few lowering to deal with things that used to be dealt
with inline when creating inputs. We also move the code that fills out
the radv_shader_variant_info struct for linking purposes to
radv_shader.c, as it's no longer tied to the NIR->LLVM lowering.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>