else { guard = guard.copyRO(); }
  OPT_RegisterOperand makeNullCheckGuard(OPT_Register ref) {    OPT_RegisterOperand guard = (OPT_RegisterOperand)_ncGuards.get(ref);    if (guard == null) {      guard = temps.makeTempValidation();      _ncGuards.put(ref, guard.copyRO());    }    return guard;  }