glsl: Use glsl_type C helpers

Acked-by: Jesse Natalie <jenatali@microsoft.com>
Acked-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26707>

src/compiler/glsl/opt_dead_code.cpp

@@ -140,7 +140,7 @@ do_dead_code(exec_list *instructions)
                }
             }
 
-            if (entry->var->type->is_subroutine())
+            if (glsl_type_is_subroutine(entry->var->type))
                continue;
          }