message: fix crash in loop dot dump when unrolling is enabled
  Reported by Jakub Kubik
diff: @@ -878,6 +878,7 @@ void CFG::printDot(ostream &os) const {
  // traverse the cfg in reverse postorder to build dominators.
  void DomTree::buildDominators(const CFG &cfg) {
  // initialization
  + // +1 for the sink BB
  unsigned i = f.getBBs().size() + 1;
  for (auto &b : f.getBBs()) {
  doms.emplace(b, *b).first->second.order = --i;
  @@ -966,7 +967,8 @@ void DomTree::printDot(ostream &os) const {
  void LoopAnalysis::getDepthFirstSpanningTree() {
  - unsigned bb_count = f.getBBs().size();
  + // +1 to account for the sink BB
  + unsigned bb_count = f.getBBs().size() + 1;
  node.resize(bb_count, nullptr);
  last.resize(bb_count, -1u);
  @@ -998,7 +1000,8 @@ void LoopAnalysis::getDepthFirstSpanningTree() {
  // Havlak, P. (1997). Nesting of reducible and irreducible loops.
  void LoopAnalysis::run() {
  getDepthFirstSpanningTree();
  - unsigned bb_count = f.getBBs().size();
  + // +1 to account for the sink BB
  + unsigned bb_count = f.getBBs().size() + 1;
  auto isAncestor = [this](unsigned w, unsigned v) -> bool {
  return w <= v && v <= last[w];
  