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

src/compiler/loop-analysis.h

Index: src/compiler/loop-analysis.h
diff --git a/src/compiler/loop-analysis.h b/src/compiler/loop-analysis.h
index cd6cd98c331192829a41b6038293de000b53baf6..71f946113d41dfb725abfeca98ac5c880033042e 100644
--- a/src/compiler/loop-analysis.h
+++ b/src/compiler/loop-analysis.h
@@ -89,6 +89,9 @@ class LoopTree : public ZoneObject {
                      &loop_nodes_[0] + loop->body_start_);
   }
 
+  // Return the header control node for a loop.
+  Node* HeaderNode(Loop* loop);
+
   // Return a range which can iterate over the body nodes of {loop}.
   NodeRange BodyNodes(Loop* loop) {
     return NodeRange(&loop_nodes_[0] + loop->body_start_,