// `if_op_segment` refers to the segment of IfRegions, `new_if_op` refers to the
// new merged IfRegion, `return_indices` refers to the indices to be kept in new
// merged IfRegion.
void ReplaceExternalUsage(
    llvm::SmallVector<TF::IfRegionOp, 8>& if_op_segment,
    TF::IfRegionOp new_if_op,
    llvm::SmallVector<llvm::SmallVector<int, 4>>& return_indices) {
  int new_return_index = 0;