message: Mark Switch node output as seen in CondContext's parent contexts.
diff: @@ -1831,7 +1831,15 @@ class CondContext(ControlFlowContext):
  result.op._set_control_flow_context(self)
  # pylint: enable=protected-access
  - self._values.add(result.name)
  + # Mark Switch output as seen by this context and any outer contexts,
  + # just like what we do for normal op outputs in _AddOpInternal() below.
  + ctxt = self
  + while ctxt is not None:
  + # pylint: disable=protected-access
  + ctxt._values.add(result.name)
  + ctxt = ctxt._outer_context
  + # pylint: enable=protected-access
  +
  self._external_values[val.name] = result
  return result
  