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

src/compiler/x64/instruction-selector-x64.cc

Index: src/compiler/x64/instruction-selector-x64.cc
diff --git a/src/compiler/x64/instruction-selector-x64.cc b/src/compiler/x64/instruction-selector-x64.cc
index b6dd082c3907023dd7a284d38b1f3fe81506563b..08121bda0cd944507fd123633bf9288c7518cc9d 100644
--- a/src/compiler/x64/instruction-selector-x64.cc
+++ b/src/compiler/x64/instruction-selector-x64.cc
@@ -572,6 +572,12 @@ void InstructionSelector::VisitWord64Ror(Node* node) {
 }
 
 
+void InstructionSelector::VisitWord64Clz(Node* node) {
+  X64OperandGenerator g(this);
+  Emit(kX64Lzcnt, g.DefineAsRegister(node), g.Use(node->InputAt(0)));
+}
+
+
 void InstructionSelector::VisitWord32Clz(Node* node) {
   X64OperandGenerator g(this);
   Emit(kX64Lzcnt32, g.DefineAsRegister(node), g.Use(node->InputAt(0)));