intel/compiler: add a new pass to lower shading rate into HW format

Rework:
 * Jason: Modernize brw_nir_lower_shading_rate_output:
   1. Use nir_shader_instructions_pass()
   2. Use *_imm builder helpers.
   3. Use nir_intrinsic_base() instead of ->const_index[0]

v2: Also lower loads (Caio)

v3: Update stage check to trigger lowering (Caio)

v4: Assert on != MESH (Caio)

v5: Fixup instruction insertion (Caio)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13739>

src/intel/compiler/brw_nir.h

@@ -120,6 +120,8 @@ bool brw_nir_lower_conversions(nir_shader *nir);
 
 bool brw_nir_lower_scoped_barriers(nir_shader *nir);
 
+bool brw_nir_lower_shading_rate_output(nir_shader *nir);
+
 bool brw_nir_lower_storage_image(nir_shader *nir,
                                  const struct intel_device_info *devinfo);