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

src/compiler/raw-machine-assembler.h

Index: src/compiler/raw-machine-assembler.h
diff --git a/src/compiler/raw-machine-assembler.h b/src/compiler/raw-machine-assembler.h
index 4a414b7dbf2ab75193cbd2f5e1ba11e9039fef6a..5c7123da3d4fa372c098dd3dcd9a8c513c45b6ad 100644
--- a/src/compiler/raw-machine-assembler.h
+++ b/src/compiler/raw-machine-assembler.h
@@ -233,6 +233,7 @@ class RawMachineAssembler {
   Node* Word64Ror(Node* a, Node* b) {
     return AddNode(machine()->Word64Ror(), a, b);
   }
+  Node* Word64Clz(Node* a) { return AddNode(machine()->Word64Clz(), a); }
   Node* Word64Equal(Node* a, Node* b) {
     return AddNode(machine()->Word64Equal(), a, b);
   }