Optimize RefCell refcount tracking

src/test/compile-fail/not-panic-safe-2.rs

@@ -19,5 +19,5 @@ fn assert<T: UnwindSafe + ?Sized>() {}
 fn main() {
     assert::<Rc<RefCell<i32>>>();
     //~^ ERROR the type `std::cell::UnsafeCell<i32>` may contain interior mutability and a
-    //~| ERROR the type `std::cell::UnsafeCell<usize>` may contain interior mutability and a
+    //~| ERROR the type `std::cell::UnsafeCell<isize>` may contain interior mutability and a
 }

src/test/compile-fail/not-panic-safe-3.rs

@@ -19,5 +19,5 @@ fn assert<T: UnwindSafe + ?Sized>() {}
 fn main() {
     assert::<Arc<RefCell<i32>>>();
     //~^ ERROR the type `std::cell::UnsafeCell<i32>` may contain interior mutability and a
-    //~| ERROR the type `std::cell::UnsafeCell<usize>` may contain interior mutability and a
+    //~| ERROR the type `std::cell::UnsafeCell<isize>` may contain interior mutability and a
 }

src/test/compile-fail/not-panic-safe-4.rs

@@ -18,5 +18,5 @@ fn assert<T: UnwindSafe + ?Sized>() {}
 fn main() {
     assert::<&RefCell<i32>>();
     //~^ ERROR the type `std::cell::UnsafeCell<i32>` may contain interior mutability and a
-    //~| ERROR the type `std::cell::UnsafeCell<usize>` may contain interior mutability and a
+    //~| ERROR the type `std::cell::UnsafeCell<isize>` may contain interior mutability and a
 }

src/test/compile-fail/not-panic-safe-6.rs

@@ -18,5 +18,5 @@ fn assert<T: UnwindSafe + ?Sized>() {}
 fn main() {
     assert::<*mut RefCell<i32>>();
     //~^ ERROR the type `std::cell::UnsafeCell<i32>` may contain interior mutability and a
-    //~| ERROR the type `std::cell::UnsafeCell<usize>` may contain interior mutability and a
+    //~| ERROR the type `std::cell::UnsafeCell<isize>` may contain interior mutability and a
 }