Fix infinite loops in cycle finder.

cycle_finder/src/main/java/com/google/devtools/cyclefinder/CycleFinder.java

@@ -220,6 +220,10 @@ private void handleStronglyConnectedComponent(ReferenceGraph subgraph) {
       TypeNode root = Iterables.getFirst(unusedTypes, null);
       assert root != null;
       List<Edge> cycle = subgraph.findShortestCycle(root);
+      if (cycle.isEmpty()) {
+        unusedTypes.remove(root);
+        continue;
+      }
       if (shouldAddCycle(cycle)) {
         cycles.add(cycle);
       }

cycle_finder/src/main/java/com/google/devtools/cyclefinder/GraphBuilder.java

@@ -135,8 +135,9 @@ private void addSubtypeEdges() {
 
   private void addSuperclassEdges() {
     for (TypeNode type : allTypes.values()) {
+      Set<TypeNode> visited = new HashSet<>();
       TypeNode superclassNode = superclasses.get(type);
-      while (superclassNode != null) {
+      while (superclassNode != null && visited.add(superclassNode)) {
         for (Edge e : graph.getEdges(superclassNode)) {
           addEdge(Edge.newSuperclassEdge(e, type, superclassNode));
         }

cycle_finder/src/main/java/com/google/devtools/cyclefinder/ReferenceGraph.java

@@ -67,6 +67,9 @@ public List<Edge> findShortestCycle(TypeNode root) {
     Set<TypeNode> visited = new HashSet<>();
     List<TypeNode> toVisit = Lists.newArrayList(root);
     outer: while (true) {
+      if (toVisit.isEmpty()) {
+        return Collections.emptyList();
+      }
       List<TypeNode> visitNext = new ArrayList<>();
       for (TypeNode source : toVisit) {
         visited.add(source);