Function get_or_split_edge 

fn get_or_split_edge<'tcx>(
    patcher: &mut MirPatch<'tcx>,
    body: &Body<'tcx>,
    split_edges: &mut FxHashMap<(BasicBlock, BasicBlock), BasicBlock>,
    pred: BasicBlock,
    succ: BasicBlock,
) -> BasicBlock

Helper function to split a critical edge if necessary.