explain why we don't need to run type-checker when NLL is enabled

src/librustc_mir/borrow_check/nll/type_check/mod.rs

@@ -1585,6 +1585,8 @@ impl MirPass for TypeckMir {
         let id = tcx.hir.as_local_node_id(def_id).unwrap();
         debug!("run_pass: {:?}", def_id);
 
+        // When NLL is enabled, the borrow checker runs the typeck
+        // itself, so we don't need this MIR pass anymore.
         if tcx.sess.nll() {
             return;
         }