nir: Add alpha_to_coverage lowering pass

Importing this pass from fs_visitor::emit_alpha_to_coverage_workaround()
in intel/compiler.

v2 (Caio Marcelo de Oliveira Filho):
- Track store output and sample mask instruction
- Nest math insturction for more readability
- Bail out early if no gl_SampleMask

v3: (Caio Marcelo de Oliveira Filho):
- Do math instructions after instruction block
- Restructure code
- Move pass under src/intel/compiler

v4: (Caio Marcelo de Oliveira Filho):
- Organize dither mask calculation

Signed-off-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>

src/intel/compiler/brw_nir.h

@@ -102,6 +102,7 @@ brw_nir_link_shaders(const struct brw_compiler *compiler,
 
 bool brw_nir_lower_cs_intrinsics(nir_shader *nir,
                                  unsigned dispatch_width);
+void brw_nir_lower_alpha_to_coverage(nir_shader *shader);
 void brw_nir_lower_legacy_clipping(nir_shader *nir,
                                    int nr_userclip_plane_consts,
                                    struct brw_stage_prog_data *prog_data);