compiler, intel: Add gl_shader_stage_is_mesh()

And replace the previous Intel-specific function.

Reviewed-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14823>

src/intel/compiler/brw_fs_nir.cpp

@@ -160,7 +160,7 @@ emit_system_values_block(nir_block *block, fs_visitor *v)
          /* For Task/Mesh, draw_id will be handled later in
           * nir_emit_mesh_task_intrinsic().
           */
-         if (!brw_shader_stage_is_mesh(v->stage))
+         if (!gl_shader_stage_is_mesh(v->stage))
             unreachable("should be lowered by brw_nir_lower_vs_inputs().");
          break;