First steps towards TF/XLA control flow lowering: functional if lowering.

 - Implement support for the TensorFlow 'If' op, the first TF op definition.
 - Fill in some missing basic infra, including the ability to split a basic block, the ability to create a branch with operands, etc.
 - Implement basic lowering for some simple forms of If, where the condition is a zero-D bool tensor and when all the types line up.  Future patches will generalize this.

There is still much to be done here.  I'd like to get some example graphs coming from the converter to play with to direct this work.

PiperOrigin-RevId: 210198760