Replace NoLandingPad with another alive Visitor

The broken reference to the non-existing `NoLandingPad` is replaced by `LocalUseCounter` to fix this document.

Author: momvart

src/mir/visitor.md

@@ -37,12 +37,10 @@ code that will execute whenever a `foo` is found. If you want to
 recursively walk the contents of the `foo`, you then invoke the
 `super_foo` method. (NB. You never want to override `super_foo`.)
 
-A very simple example of a visitor can be found in [`NoLandingPads`].
-That visitor doesn't even require any state: it just visits all
-terminators and removes their `unwind` successors.
+A very simple example of a visitor can be found in [`LocalUseCounter`].
+By implementing `visit_local` method, this visitor counts how many tims each local is used.
 
-<!--- TODO: Change NoLandingPads. [#1232](https://github.com/rust-lang/rustc-dev-guide/issues/1232) -->
-[`NoLandingPads`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/no_landing_pads/struct.NoLandingPads.html
+[`LocalUseCounter`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/simplify_try/struct.LocalUseCounter.html
 
 ## Traversal