`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

```mlir
    %0 = "tf.IfRegion"(%arg0) ( {

LogicalResult IfOp::verifySymbolUses(SymbolTableCollection& symbol_table) {
  auto branch_name = [](unsigned index) -> std::string {
    return index == 0 ? "'then_branch'" : "'else_branch'";
  };
  return VerifyCaseOrIfOpBranchFunctions(
      symbol_table, *this, {getThenBranchAttr(), getElseBranchAttr()},
      branch_name);
}

  // arguments: the condition returns a bool and the two branches must return
  // the same type, which is also the return of the tf.If.
  %value = "tf.If”(%added, %mul)
             {cond: @cond_func, true_branch: @func_foo, false_branch: @func_bar}
                 : (tensor<*xf32>, tensor<*xf32>) -> tensor<*xf32>

  return %value : tensor<*xf32>
}
```

## TensorFlow Executor Dialect