Reduce field type tracking to a single stable map per field.

src/types.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <iomanip>
 
 #include "src/types.h"
 
 #include "src/ostreams.h"
 #include "src/types-inl.h"
@@ skipping 557 matching lines @@
     }
   }
   return this->Is(that);
 }
 
 
 // Check if [this] contains only (currently) stable classes.
 template<class Config>
 bool TypeImpl<Config>::NowStable() {
   DisallowHeapAllocation no_allocation;
-  for (Iterator<i::Map> it = this->Classes(); !it.Done(); it.Advance()) {
-    if (!it.Current()->is_stable()) return false;
-  }
-  return true;
+  return !this->IsClass() || this->AsClass()->Map()->is_stable();
 }
 
 
 // Check if [this] and [that] overlap.
 template<class Config>
 bool TypeImpl<Config>::Maybe(TypeImpl* that) {
   DisallowHeapAllocation no_allocation;
 
   // Take care of the representation part (and also approximate
   // the semantic part).
@@ skipping 768 matching lines @@
 template class TypeImpl<HeapTypeConfig>::Iterator<i::Object>;
 
 template TypeImpl<ZoneTypeConfig>::TypeHandle
   TypeImpl<ZoneTypeConfig>::Convert<HeapType>(
     TypeImpl<HeapTypeConfig>::TypeHandle, TypeImpl<ZoneTypeConfig>::Region*);
 template TypeImpl<HeapTypeConfig>::TypeHandle
   TypeImpl<HeapTypeConfig>::Convert<Type>(
     TypeImpl<ZoneTypeConfig>::TypeHandle, TypeImpl<HeapTypeConfig>::Region*);
 
 } }  // namespace v8::internal