[turbofan] Use heavy-handed graph duplication to do loop peeling for OSR.

src/compiler/loop-analysis.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/loop-analysis.h"
 
 #include "src/compiler/graph.h"
 #include "src/compiler/node.h"
 #include "src/compiler/node-marker.h"
 #include "src/compiler/node-properties.h"
@@ skipping 444 matching lines @@
   LoopTree* loop_tree =
       new (graph->zone()) LoopTree(graph->NodeCount(), graph->zone());
   LoopFinderImpl finder(graph, loop_tree, zone);
   finder.Run();
   if (FLAG_trace_turbo_graph) {
     finder.Print();
   }
   return loop_tree;
 }
 
+
+Node* LoopTree::HeaderNode(Loop* loop) {
+  Node* first = *HeaderNodes(loop).begin();
+  if (first->opcode() == IrOpcode::kLoop) return first;
+  DCHECK(IrOpcode::IsPhiOpcode(first->opcode()));
+  Node* header = NodeProperties::GetControlInput(first);
+  DCHECK_EQ(IrOpcode::kLoop, header->opcode());
+  return header;
+}
+
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8