opt_clean: Assert an impossible path isn't taken

passes/opt/opt_clean.cc

@@ -438,6 +438,9 @@ bool rmunused_module_signals(RTLIL::Module *module, bool purge_mode, bool verbos
 			goto delete_this_wire;
 		} else
 		if (!used_signals.check_any(s2)) {
+			// this path shouldn't be possible: this wire is used directly (otherwise it would get cleaned up above), and indirectly
+			// used wires are a superset of those used directly
+			log_assert(false);
 			// delete wires that aren't used by anything indirectly, even though other wires may alias it
 			goto delete_this_wire;
 		}