Visit mir::Constant::user_ty for completeness.

It's not necessary yet, but it may become necessary with things like lazy normalization.

compiler/rustc_middle/src/mir/type_foldable.rs

@@ -342,8 +342,8 @@ fn super_fold_with<F: TypeFolder<'tcx>>(self, folder: &mut F) -> Self {
         }
     }
     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
-        self.literal.visit_with(visitor)
-        // FIXME: should this be visiting the `user_ty`, too?
+        self.literal.visit_with(visitor)?;
+        self.user_ty.visit_with(visitor)
     }
 }