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

src/x64/assembler-x64.h

 // Copyright (c) 1994-2006 Sun Microsystems Inc.
 // All Rights Reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 // - Redistributions of source code must retain the above copyright notice,
 // this list of conditions and the following disclaimer.
 //
@@ skipping 829 matching lines @@
   }
 
   void testb(Register dst, Register src);
   void testb(Register reg, Immediate mask);
   void testb(const Operand& op, Immediate mask);
   void testb(const Operand& op, Register reg);
 
   // Bit operations.
   void bt(const Operand& dst, Register src);
   void bts(const Operand& dst, Register src);
+  void bsrq(Register dst, Register src);
+  void bsrq(Register dst, const Operand& src);
   void bsrl(Register dst, Register src);
   void bsrl(Register dst, const Operand& src);
   void bsfl(Register dst, Register src);
   void bsfl(Register dst, const Operand& src);
 
   // Miscellaneous
   void clc();
   void cld();
   void cpuid();
   void hlt();
@@ skipping 1301 matching lines @@
   Assembler* assembler_;
 #ifdef DEBUG
   int space_before_;
 #endif
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_X64_ASSEMBLER_X64_H_