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

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

Index: src/compiler/x64/code-generator-x64.cc
diff --git a/src/compiler/x64/code-generator-x64.cc b/src/compiler/x64/code-generator-x64.cc
index 071dc48337923543866065868c834d16b87df01d..897e96c1d3b99b798904c77af1f5b649b5228a5b 100644
--- a/src/compiler/x64/code-generator-x64.cc
+++ b/src/compiler/x64/code-generator-x64.cc
@@ -768,6 +768,13 @@ void CodeGenerator::AssembleArchInstruction(Instruction* instr) {
     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));