Verifier must ensure that start and end node of graph are set.

BUG=
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/444583004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22862 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/compiler/verifier.cc

@@ -224,8 +224,10 @@ GenericGraphVisit::Control Verifier::Visitor::Pre(Node* node) {
 void Verifier::Run(Graph* graph) {
   Visitor visitor(graph->zone());
 
+  CHECK_NE(NULL, graph->start());
   visitor.from_start = true;
   graph->VisitNodeUsesFromStart(&visitor);
+  CHECK_NE(NULL, graph->end());
   visitor.from_start = false;
   graph->VisitNodeInputsFromEnd(&visitor);