Set vtable in class object to null after linking.

This is follow-up work of embedding imt and vtable for
faster interface/virtual call dispatching.
Once vtable becomes embedded, the original vtable is nulled.

Change-Id: I307696657d1e283654169dbecb8f7815c42bbabc
diff --git a/runtime/mirror/art_method.cc b/runtime/mirror/art_method.cc
index 167f848..211ba1d 100644
--- a/runtime/mirror/art_method.cc
+++ b/runtime/mirror/art_method.cc
@@ -130,12 +130,11 @@
   Class* declaring_class = GetDeclaringClass();
   Class* super_class = declaring_class->GetSuperClass();
   uint16_t method_index = GetMethodIndex();
-  ObjectArray<ArtMethod>* super_class_vtable = super_class->GetVTable();
   ArtMethod* result = NULL;
   // Did this method override a super class method? If so load the result from the super class'
   // vtable
-  if (super_class_vtable != NULL && method_index < super_class_vtable->GetLength()) {
-    result = super_class_vtable->Get(method_index);
+  if (super_class->HasVTable() && method_index < super_class->GetVTableLength()) {
+    result = super_class->GetVTableEntry(method_index);
   } else {
     // Method didn't override superclass method so search interfaces
     if (IsProxyMethod()) {