iris: Add a comment for iris_uncompiled_shader::nir

This would have saved me a few minutes searching around.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11229>

src/gallium/drivers/iris/iris_context.h

@@ -371,6 +371,11 @@ enum iris_predicate_state {
  * See iris_compiled_shader, which represents a compiled shader variant.
  */
 struct iris_uncompiled_shader {
+   /**
+    * NIR for the shader.
+    *
+    * Even for shaders that originate as TGSI, this pointer will be non-NULL.
+    */
    struct nir_shader *nir;
 
    struct pipe_stream_output_info stream_output;