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

src/compiler/arm64/code-generator-arm64.cc

 // Copyright 2014 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.
 
 #include "src/compiler/code-generator.h"
 
 #include "src/arm64/frames-arm64.h"
 #include "src/arm64/macro-assembler-arm64.h"
 #include "src/compiler/code-generator-impl.h"
 #include "src/compiler/gap-resolver.h"
@@ skipping 756 matching lines @@
     case kArm64Poke: {
       Operand operand(i.InputInt32(1) * kPointerSize);
       __ Poke(i.InputRegister(0), operand);
       break;
     }
     case kArm64PokePair: {
       int slot = i.InputInt32(2) - 1;
       __ PokePair(i.InputRegister(1), i.InputRegister(0), slot * kPointerSize);
       break;
     }
+    case kArm64Clz:
+      __ Clz(i.OutputRegister64(), i.InputRegister64(0));
+      break;
     case kArm64Clz32:
       __ Clz(i.OutputRegister32(), i.InputRegister32(0));
       break;
     case kArm64Cmp:
       __ Cmp(i.InputOrZeroRegister64(0), i.InputOperand(1));
       break;
     case kArm64Cmp32:
       __ Cmp(i.InputOrZeroRegister32(0), i.InputOperand2_32(1));
       break;
     case kArm64Cmn:
@@ skipping 673 matching lines @@
       padding_size -= kInstructionSize;
     }
   }
 }
 
 #undef __
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8