New hydrogen instruction to reduce cost of growing an array on keyed stores.

src/interface-descriptors.h

Index: src/interface-descriptors.h
diff --git a/src/interface-descriptors.h b/src/interface-descriptors.h
index 6ce5faacf44df98d317ded079e1e2d5f2483372b..239469ba0cf0ed97396fc3cda313ed34e85d77f7 100644
--- a/src/interface-descriptors.h
+++ b/src/interface-descriptors.h
@@ -531,10 +531,9 @@ class GrowArrayElementsDescriptor : public CallInterfaceDescriptor {
  public:
   DECLARE_DESCRIPTOR(GrowArrayElementsDescriptor, CallInterfaceDescriptor)
 
-  enum RegisterInfo { kObjectIndex, kKeyIndex, kCapacityIndex };
+  enum RegisterInfo { kObjectIndex, kKeyIndex };
   static const Register ObjectRegister();
   static const Register KeyRegister();
-  static const Register CapacityRegister();
 };
 
 #undef DECLARE_DESCRIPTOR