VM: Reland Inline instance object hash code into object header on 64bit.

runtime/vm/heap.cc

Index: runtime/vm/heap.cc
diff --git a/runtime/vm/heap.cc b/runtime/vm/heap.cc
index 975d0d55c27a38cdea105418f05c13953f0103a2..282272cc9f57504640381e4674456be9a9666eaa 100644
--- a/runtime/vm/heap.cc
+++ b/runtime/vm/heap.cc
@@ -665,11 +665,12 @@ int64_t Heap::PeerCount() const {
   return new_weak_tables_[kPeers]->count() + old_weak_tables_[kPeers]->count();
 }
 
-
+#if !defined(HASH_IN_OBJECT_HEADER)
 int64_t Heap::HashCount() const {
   return new_weak_tables_[kHashes]->count() +
          old_weak_tables_[kHashes]->count();
 }
+#endif
 
 
 int64_t Heap::ObjectIdCount() const {