Add and use generics.is_empty() and generics.is_own_empty, rather than using generics' attributes

compiler/rustc_monomorphize/src/polymorphize.rs

@@ -51,7 +51,7 @@ fn unused_generic_params<'tcx>(
     debug!(?generics);
 
     // Exit early when there are no parameters to be unused.
-    if generics.count() == 0 {
+    if generics.is_empty() {
         return UnusedGenericParams::new_all_used();
     }