message: Graph construction: skip testing for duplicate edges during source/sink setup.
  When fixing up source/sink edges, we are only adding missing edges.  Skip the
  test for duplicate edges in this case, which avoids a linear traversal over
  sink edges.
diff: @@ -222,11 +222,12 @@ bool FixupSourceAndSinkEdges(Graph* g) {
  bool changed = false;
  for (Node* n : g->nodes()) {
  if (!n->IsSource() && n->in_edges().empty()) {
  - g->AddControlEdge(g->source_node(), n);
  + g->AddControlEdge(g->source_node(), n,
  + true /* skip test for duplicates */);
  changed = true;
  }
  if (!n->IsSink() && n->out_edges().empty()) {
  - g->AddControlEdge(n, g->sink_node());
  + g->AddControlEdge(n, g->sink_node(), true /* skip test for duplicates */);
  changed = true;
  }
  }
  