Allow HPhis to have an invalid merge index.

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index f977a750d946a6b24d657b08229fc4a238c368b2..25bcbda840bef5a41c031943710b7450c46698b6 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -3020,7 +3020,7 @@ class HPhi: public HValue {
       non_phi_uses_[i] = 0;
       indirect_uses_[i] = 0;
     }
-    ASSERT(merged_index >= 0);
+    ASSERT(merged_index >= 0 || merged_index == kInvalidMergedIndex);
     SetFlag(kFlexibleRepresentation);
     SetFlag(kAllowUndefinedAsNaN);
   }
@@ -3043,6 +3043,7 @@ class HPhi: public HValue {
   bool HasRealUses();
 
   bool IsReceiver() const { return merged_index_ == 0; }
+  bool HasMergedIndex() const { return merged_index_ != kInvalidMergedIndex; }
 
   int merged_index() const { return merged_index_; }
 
@@ -3105,6 +3106,9 @@ class HPhi: public HValue {
 
   void SimplifyConstantInputs();
 
+  // Marker value representing an invalid merge index.
+  static const int kInvalidMergedIndex = -1;
+
  protected:
   virtual void DeleteFromGraph();
   virtual void InternalSetOperandAt(int index, HValue* value) {