cxxrtl: ignore $input_port

backends/cxxrtl/cxxrtl_backend.cc

@@ -1518,6 +1518,7 @@ struct CxxrtlWorker {
 				f << (cell->getParam(ID::CLR_POLARITY).as_bool() ? "" : ".bit_not()") << ");\n";
 			}
 		// Internal cells
+		} else if (cell->type == ID($input_port)) {
 		} else if (is_internal_cell(cell->type)) {
 			log_cmd_error("Unsupported internal cell `%s'.\n", cell->type);
 		// User cells