`tf.IfRegion` and `tf.While` is transformed to `tf.WhileRegion`.

    For example, this functional operation

    ```mlir
      %0 = "tf.If"(%arg0, %arg1) {
        then_branch = @then_branch_func, else_branch = @else_branch_func, is_stateless = false
      } : (tensor<i1>, tensor<*xf32>) -> tensor<*xf32>
    ```

    will be transformed into this region-based operation