Introduce TF WhileOp and lower it to MLIR CFG
Introduce TF WhileOp and lower it to MLIR CFG

Also, added iterators for VariadicResults class.

TESTED with unit tests

TODOs:
- Handle non-bool condition results (similar to the IfOp)
- Use PatternRewriter
PiperOrigin-RevId: 222340376

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