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

src/compiler/x64/code-generator-x64.cc

 // 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.
 
 #include "src/compiler/code-generator.h"
 
 #include "src/compiler/code-generator-impl.h"
 #include "src/compiler/gap-resolver.h"
 #include "src/compiler/node-matchers.h"
 #include "src/compiler/osr.h"
@@ skipping 750 matching lines @@
       break;
     case kX64Sar:
       ASSEMBLE_SHIFT(sarq, 6);
       break;
     case kX64Ror32:
       ASSEMBLE_SHIFT(rorl, 5);
       break;
     case kX64Ror:
       ASSEMBLE_SHIFT(rorq, 6);
       break;
+    case kX64Lzcnt:
+      if (instr->InputAt(0)->IsRegister()) {
+        __ Lzcntq(i.OutputRegister(), i.InputRegister(0));
+      } else {
+        __ Lzcntq(i.OutputRegister(), i.InputOperand(0));
+      }
+      break;
     case kX64Lzcnt32:
       if (instr->InputAt(0)->IsRegister()) {
         __ Lzcntl(i.OutputRegister(), i.InputRegister(0));
       } else {
         __ Lzcntl(i.OutputRegister(), i.InputOperand(0));
       }
       break;
     case kX64Tzcnt32:
       if (instr->InputAt(0)->IsRegister()) {
         __ Tzcntl(i.OutputRegister(), i.InputRegister(0));
@@ skipping 1057 matching lines @@
     int padding_size = last_lazy_deopt_pc_ + space_needed - current_pc;
     __ Nop(padding_size);
   }
 }
 
 #undef __
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8