nir: Mark variable as ASSERTED to fix unused variable warning treated as error

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29290>

src/compiler/nir/nir_lower_printf.c

@@ -178,7 +178,7 @@ nir_printf_fmt(nir_builder *b,
       default:  unreachable("invalid");
       }
 
-      nir_def *def = va_arg(ap, nir_def*);
+      ASSERTED nir_def *def = va_arg(ap, nir_def*);
       assert(def->bit_size / 8 == arg_size);
 
       info->num_args++;