ac/nir: handle nir_intrinsic_load_first_vertex like base_vertex
This fixes a ton of CTS crashes. Fixes: c366f422 ("nir: Offset vertex_id by first_vertex instead of base_vertex") Signed-off-by:Samuel Pitoiset <samuel.pitoiset@gmail.com> Reviewed-by:
Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>