nir: add a pass to lower flat shading.

This takes any color or backcolor that has unspecified
shading and converts it to flat shading.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/compiler/nir/nir.h

@@ -3916,6 +3916,8 @@ void nir_lower_two_sided_color(nir_shader *shader);
 
 bool nir_lower_clamp_color_outputs(nir_shader *shader);
 
+bool nir_lower_flatshade(nir_shader *shader);
+
 void nir_lower_passthrough_edgeflags(nir_shader *shader);
 bool nir_lower_patch_vertices(nir_shader *nir, unsigned static_count,
                               const gl_state_index16 *uniform_state_tokens);