Avoid more unnecessary mk_ty calls in Ty::super_fold_with.

This speeds up several rustc-benchmarks by 1--5%.

src/librustc/ty/structural_impls.rs

@@ -481,8 +481,13 @@ impl<'tcx> TypeFoldable<'tcx> for Ty<'tcx> {
             ty::TyUint(_) | ty::TyFloat(_) | ty::TyError | ty::TyInfer(_) |
             ty::TyParam(..) | ty::TyNever => return self
         };
+
+        if self.sty == sty {
+            self
+        } else {
             folder.tcx().mk_ty(sty)
         }
+    }
 
     fn fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, folder: &mut F) -> Self {
         folder.fold_ty(*self)