Reland "In-object double fields unboxing (for 64-bit only)."

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index 9101576b7b91a3b4c9746e1ca11da7bb3eb51320..1a634aff2c42bf582d1fa52a161ae7f78972e23d 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -6943,7 +6943,9 @@ class HStoreNamedField FINAL : public HTemplateInstruction<3> {
   }
 
   bool NeedsWriteBarrier() const {
-    DCHECK(!field_representation().IsDouble() || !has_transition());
+    DCHECK(!field_representation().IsDouble() ||
+           (FLAG_unbox_double_fields && access_.IsInobject()) ||
+           !has_transition());
     if (field_representation().IsDouble()) return false;
     if (field_representation().IsSmi()) return false;
     if (field_representation().IsInteger32()) return false;