Add IsCondSwitch.

Add IsCondSwitch.

* Switch nodes are not part of the cond contexts of the tf.cond that they are the switches for, so check the contexts of the outputs of the switch to determine if a cond switch.
* Include the pivot of a cond in its cond context (there is one pivot per CondContext)
* If a cond is nested in a while loop, then the switch nodes of the cond is in the control flow context of the while loop, so only return that it is a loop switch if it isn't a cond switch.

PiperOrigin-RevId: 196015879