[TF2XLA] Keep Switch and Merge nodes in own clusters.

* Keep Switch and Merge nodes in separate clusters to avoid creating irreducible graphs;
* Merge Switch nodes with common predicates;
* Add support for if-then structure;
* Squash trivial Switch->Merge groups;
* Merge newly Merge free nodes with Switch & Merge free inputs;
* Check to see if it is a Merge node before merging to common merge node;
* Return an error if all Switches have not been replaced;
* Add test fir tf,case;

PiperOrigin-RevId: 172348729