ac/nir: assign argument param pointers in one place.

ac/nir: assign argument param pointers in one place.

Instead of having the fragile code to do a second pass, just
give the pointers you want params in to the initial code,
then call a later pass to assign them.

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