Improve GVN handling of ElementTransitions.

src/hydrogen.h

Index: src/hydrogen.h
diff --git a/src/hydrogen.h b/src/hydrogen.h
index fc4b6121048423cfdcd4764cf555f571ff362704..47b569146bb271fc81f1ce265633cd23b3d71b81 100644
--- a/src/hydrogen.h
+++ b/src/hydrogen.h
@@ -126,6 +126,7 @@ class HBasicBlock: public ZoneObject {
   int PredecessorIndexOf(HBasicBlock* predecessor) const;
   void AddSimulate(int ast_id) { AddInstruction(CreateSimulate(ast_id)); }
   void AssignCommonDominator(HBasicBlock* other);
+  void AssignLoopSuccessorDominators();
 
   void FinishExitWithDeoptimization(HDeoptimize::UseEnvironment has_uses) {
     FinishExit(CreateDeoptimize(has_uses));
@@ -149,6 +150,13 @@ class HBasicBlock: public ZoneObject {
   bool IsDeoptimizing() const { return is_deoptimizing_; }
   void MarkAsDeoptimizing() { is_deoptimizing_ = true; }
 
+  bool IsLoopSuccessorDominator() const {
+    return dominates_loop_successors_;
+  }
+  void MarkAsLoopSuccessorDominator() {
+    dominates_loop_successors_ = true;
+  }
+
   inline Zone* zone();
 
 #ifdef DEBUG
@@ -182,6 +190,22 @@ class HBasicBlock: public ZoneObject {
   HBasicBlock* parent_loop_header_;
   bool is_inline_return_target_;
   bool is_deoptimizing_;
+  bool dominates_loop_successors_;
+};
+
+
+class HPredecessorIterator BASE_EMBEDDED {
+ public:
+  explicit HPredecessorIterator(HBasicBlock* block)
+      : predecessor_list_(block->predecessors()), current_(0) { }
+
+  bool Done() { return current_ >= predecessor_list_->length(); }
+  HBasicBlock* Current() { return predecessor_list_->at(current_); }
+  void Advance() { current_++; }
+
+ private:
+  const ZoneList<HBasicBlock*>* predecessor_list_;
+  int current_;
 };