Implement bitLength intrinsic on x64.

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 623 matching lines @@
   void incq(Register reg);
   void incq(const Address& address);
   void decq(Register reg);
   void decq(const Address& address);
 
   void negl(Register reg);
   void negq(Register reg);
   void notl(Register reg);
   void notq(Register reg);
 
+  void bsrq(Register dst, Register src);
+
   void btq(Register base, Register offset);
 
   void enter(const Immediate& imm);
   void leave();
   void ret();
 
   void movmskpd(Register dst, XmmRegister src);
   void movmskps(Register dst, XmmRegister src);
 
   void sqrtsd(XmmRegister dst, XmmRegister src);
@@ skipping 547 matching lines @@
 }
 
 
 inline void Assembler::EmitOperandSizeOverride() {
   EmitUint8(0x66);
 }
 
 }  // namespace dart
 
 #endif  // VM_ASSEMBLER_X64_H_