share code between used-item-visit and mentioned-item-visit logic

Author: RalfJung

compiler/rustc_mir_transform/src/mentioned_items.rs

@@ -18,6 +18,7 @@ impl<'tcx> MirPass<'tcx> for MentionedItems {
         // 'mentioned' items, so if some item X gets compiled without optimizations but later called
         // from somewhere that has optimizations enabled, we need the `mentioned_items` of X to be
         // filled.
+        // Furthermore, even debug builds use level 1, so special-casing level 0 is just not worth it.
         true
     }