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

src/compiler/arm64/instruction-selector-arm64.cc

Index: src/compiler/arm64/instruction-selector-arm64.cc
diff --git a/src/compiler/arm64/instruction-selector-arm64.cc b/src/compiler/arm64/instruction-selector-arm64.cc
index da43b417d980fc5f2cc5d59abe3ea69602cdeeab..c872e9619787072a25da41e846b384f26906bcad 100644
--- a/src/compiler/arm64/instruction-selector-arm64.cc
+++ b/src/compiler/arm64/instruction-selector-arm64.cc
@@ -921,6 +921,12 @@ void InstructionSelector::VisitWord64Ror(Node* node) {
 }
 
 
+void InstructionSelector::VisitWord64Clz(Node* node) {
+  Arm64OperandGenerator g(this);
+  Emit(kArm64Clz, g.DefineAsRegister(node), g.UseRegister(node->InputAt(0)));
+}
+
+
 void InstructionSelector::VisitWord32Clz(Node* node) {
   Arm64OperandGenerator g(this);
   Emit(kArm64Clz32, g.DefineAsRegister(node), g.UseRegister(node->InputAt(0)));