[CSSPGO][SCCIterator] Fix a non-determinism in scc_member_iterator

[hoy]

Summary

Previously we initialed the work queue with MST roots based on NodeInfoMap which is an unordered map. This could cause a non-determinism. I'm fixing this by initializing the queue based on SortedEdges.

I don't see any performance move with this change. However this helps debugging.

[wenlei]

lgtm, thanks.

llvm/include/llvm/ADT/SCCIterator.h

scc_member_iterator<GraphT, GT>::scc_member_iterator(
   // Do BFS on MST, starting from nodes that have no incoming edge. These nodes
   // are "roots" of the MST forest. This ensures that nodes are visited before
   // their decsendents are, thus ensures hot edges are processed before cold
   // edges, based on how MST is computed.
   for (const auto *Edge : MSTEdges)
     NodeInfoMap[Edge->Target].Visited = false;
 
   std::queue<NodeType *> Queue;
-  for (auto &Node : NodeInfoMap)
-    if (Node.second.Visited)
-      Queue.push(Node.first);
+  // Initialze the queue with MST roots. Note that walking through SortedEdges
+  // instead of NodeInfoMap ensures an ordered deterministic push.
+  for (auto *Edge : SortedEdges) {
+    if (NodeInfoMap[Edge->Source].Visited) {
+      Queue.push(Edge->Source);
+      NodeInfoMap[Edge->Source].Visited = false;
+    }
+  }
 
   while (!Queue.empty()) {
     auto *Node = Queue.front();
     Queue.pop();
     Nodes.push_back(Node);
     for (auto &Edge : Node->Edges) {
       if (MSTEdges.count(&Edge) && !NodeInfoMap[Edge.Target].Visited) {
         NodeInfoMap[Edge.Target].Visited = true;