nir: Add helpers for getting input/output intrinsic sources.

nir: Add helpers for getting input/output intrinsic sources.

With the many variants of IO intrinsics, particular sources are often in
different locations.  It's convenient to say "give me the indirect
offset" or "give me the vertex index" and have it just work, without
having to think about exactly which kind of intrinsic you have.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>