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

src/compiler/opcodes.h

 // Copyright 2013 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_COMPILER_OPCODES_H_
 #define V8_COMPILER_OPCODES_H_
 
 #include <iosfwd>
 
 // Opcodes for control operators.
@@ skipping 222 matching lines @@
   V(Word32Clz)                  \
   V(Word32Ctz)                  \
   V(Word32Popcnt)               \
   V(Word64And)                  \
   V(Word64Or)                   \
   V(Word64Xor)                  \
   V(Word64Shl)                  \
   V(Word64Shr)                  \
   V(Word64Sar)                  \
   V(Word64Ror)                  \
+  V(Word64Clz)                  \
   V(Int32Add)                   \
   V(Int32AddWithOverflow)       \
   V(Int32Sub)                   \
   V(Int32SubWithOverflow)       \
   V(Int32Mul)                   \
   V(Int32MulHigh)               \
   V(Int32Div)                   \
   V(Int32Mod)                   \
   V(Uint32Div)                  \
   V(Uint32Mod)                  \
@@ skipping 121 matching lines @@
   }
 };
 
 std::ostream& operator<<(std::ostream&, IrOpcode::Value);
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_OPCODES_H_