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 6f69f3c8ceb7c32071522b69b88a8cb5f90c9273..63e814a0c4a2131ac16b2a9424a90f37c329402d 100644
--- a/src/interface-descriptors.h
+++ b/src/interface-descriptors.h
@@ -545,10 +545,9 @@ 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