Use rep_movsb for copying one-byte strings in substring intrinsic.

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 VM_ASSEMBLER_X64_H_
 #define VM_ASSEMBLER_X64_H_
 
 #ifndef VM_ASSEMBLER_H_
 #error Do not include assembler_x64.h directly; use assembler.h instead.
 #endif
@@ skipping 352 matching lines @@
 
   void movq(Register dst, const Immediate& imm);
   void movq(Register dst, Register src);
   void movq(Register dst, const Address& src);
   void movq(const Address& dst, Register src);
   void movq(const Address& dst, const Immediate& imm);
 
   void movsxd(Register dst, Register src);
   void movsxd(Register dst, const Address& src);
 
+  void rep_movsb();
+
   void leaq(Register dst, const Address& src);
 
   void movss(XmmRegister dst, const Address& src);
   void movss(const Address& dst, XmmRegister src);
   void movss(XmmRegister dst, XmmRegister src);
 
   void movd(XmmRegister dst, Register src);
   void movd(Register dst, XmmRegister src);
 
   void addss(XmmRegister dst, XmmRegister src);
@@ skipping 524 matching lines @@
 }
 
 
 inline void Assembler::EmitOperandSizeOverride() {
   EmitUint8(0x66);
 }
 
 }  // namespace dart
 
 #endif  // VM_ASSEMBLER_X64_H_