intel/rt: Add lowering functions for each ray-tracing stage

intel/rt: Add lowering functions for each ray-tracing stage

These will eventually contain per-stage lowering for various ray-tracing
things.  This is separate from brw_nir_lower_rt_intrinsics because, for
reasons that will become apparent later, brw_nir_lower_rt_intrinsics has
to be run very late in the compile process, right before brw_compile_bs.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7356>