Implemented the Word64Clz TurboFan operator for x64, arm64, and mips64.

src/x64/macro-assembler-x64.h

 // Copyright 2012 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.
 
 #ifndef V8_X64_MACRO_ASSEMBLER_X64_H_
 #define V8_X64_MACRO_ASSEMBLER_X64_H_
 
 #include "src/assembler.h"
 #include "src/bailout-reason.h"
 #include "src/base/flags.h"
@@ skipping 992 matching lines @@
   void CallSelf() {
     Handle<Code> self(reinterpret_cast<Code**>(CodeObject().location()));
     Call(self, RelocInfo::CODE_TARGET);
   }
 
   // Non-SSE2 instructions.
   void Pextrd(Register dst, XMMRegister src, int8_t imm8);
   void Pinsrd(XMMRegister dst, Register src, int8_t imm8);
   void Pinsrd(XMMRegister dst, const Operand& src, int8_t imm8);
 
+  void Lzcntq(Register dst, Register src);
+  void Lzcntq(Register dst, const Operand& src);
+
   void Lzcntl(Register dst, Register src);
   void Lzcntl(Register dst, const Operand& src);
 
   void Tzcntl(Register dst, Register src);
   void Tzcntl(Register dst, const Operand& src);
 
   void Popcntl(Register dst, Register src);
   void Popcntl(Register dst, const Operand& src);
 
   // Non-x64 instructions.
@@ skipping 694 matching lines @@
   }                                                                          \
   masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_X64_MACRO_ASSEMBLER_X64_H_