// token 'token', used for If op, and erase the argument.
    // Note that 'RewriteControlFlowOpRegion' sets the token, used for the first
    // operation of region_idx'th region, to the dummy block-argument. As we
    // erase that argument, we also need to make sure that the token used for
    // the next operation is set to 'token'.
    RewriteControlFlowOpRegion(builder, region_if, *region_idx,