Use off-heap data for class check instructions

runtime/vm/assembler_x64.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef RUNTIME_VM_ASSEMBLER_X64_H_
 #define RUNTIME_VM_ASSEMBLER_X64_H_
 
 #ifndef RUNTIME_VM_ASSEMBLER_H_
 #error Do not include assembler_x64.h directly; use assembler.h instead.
 #endif
@@ skipping 834 matching lines @@
    */
   void SmiTag(Register reg) { addq(reg, reg); }
 
   void SmiUntag(Register reg) { sarq(reg, Immediate(kSmiTagSize)); }
 
   void BranchIfNotSmi(Register reg, Label* label) {
     testq(reg, Immediate(kSmiTagMask));
     j(NOT_ZERO, label);
   }
 
+  void BranchIfSmi(Register reg, Label* label) {
+    testq(reg, Immediate(kSmiTagMask));
+    j(ZERO, label);
+  }
+
   void Align(int alignment, intptr_t offset);
   void Bind(Label* label);
   void Jump(Label* label) { jmp(label); }
 
   void Comment(const char* format, ...) PRINTF_ATTRIBUTE(2, 3);
   static bool EmittingComments();
 
   const Code::Comments& GetCodeComments() const;
 
   // Address of code at offset.
@@ skipping 279 matching lines @@
 }
 
 
 inline void Assembler::EmitOperandSizeOverride() {
   EmitUint8(0x66);
 }
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_ASSEMBLER_X64_H_