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

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

Index: src/compiler/arm64/code-generator-arm64.cc
diff --git a/src/compiler/arm64/code-generator-arm64.cc b/src/compiler/arm64/code-generator-arm64.cc
index 511e08c02d9df37c538a2862580a52f60a04547b..26a6a3526c6919ddac5919ce8a5b180097318733 100644
--- a/src/compiler/arm64/code-generator-arm64.cc
+++ b/src/compiler/arm64/code-generator-arm64.cc
@@ -774,6 +774,9 @@ void CodeGenerator::AssembleArchInstruction(Instruction* instr) {
       __ 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;