Add If op rewriter.

Add If op rewriter.

* Add attribute to If op to indicate if lowering to switch-merge form is
  needed;
* Add initial version of If op rewriter than transforms a If op into
  switch/merge nodes (as would have been constructed via tf.cond) if the If op
  has the lowering attribute set.
  - The pass is not ready for general use and, for example, does not support
    reference data types.

PiperOrigin-RevId: 196563421