message: fix masking for single instance in vcall_jit_record
diff: @@ -155,7 +155,8 @@ Result vcall_jit_record(const char *name, const Func &func,
  if constexpr (IsVoid)
  func(inst, args...);
  else
  - result = func(inst, args...);
  + result = select(neq(self, nullptr), func(inst, args...),
  + zero<Result_2>());
  } else {
  result = vcall_impl<Result_2>(
  name, n_inst, func, self,
  