radv/ac: add initial pre-pass for shader info gathering

radv/ac: add initial pre-pass for shader info gathering

There is some radv specific info we need to gather from shaders
before we get into converting nir->llvm, so we can make
better decisions especially around user sgpr allocation.

This is just an initial placeholder to gather if sample positions
are required in the frag shader.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>