Improve dead phi elimination....

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
===================================================================
--- src/hydrogen-instructions.h	(revision 7081)
+++ src/hydrogen-instructions.h	(working copy)
@@ -1800,7 +1800,8 @@
   explicit HPhi(int merged_index)
       : inputs_(2),
         merged_index_(merged_index),
-        phi_id_(-1) {
+        phi_id_(-1),
+        is_live_(false) {
     for (int i = 0; i < Representation::kNumRepresentations; i++) {
       non_phi_uses_[i] = 0;
       indirect_uses_[i] = 0;
@@ -1834,6 +1835,7 @@
   virtual HValue* OperandAt(int index) { return inputs_[index]; }
   HValue* GetRedundantReplacement();
   void AddInput(HValue* value);
+  bool HasRealUses();
 
   bool IsReceiver() { return merged_index_ == 0; }
 
@@ -1872,6 +1874,8 @@
     return indirect_uses_[Representation::kDouble];
   }
   int phi_id() { return phi_id_; }
+  bool is_live() { return is_live_; }
+  void set_is_live(bool b) { is_live_ = b; }
 
  protected:
   virtual void DeleteFromGraph();
@@ -1886,6 +1890,7 @@
   int non_phi_uses_[Representation::kNumRepresentations];
   int indirect_uses_[Representation::kNumRepresentations];
   int phi_id_;
+  bool is_live_;
 };