Do not traverse RHS of assignment twice for unused variables lint

walk_expr includes call to visit_expr for subexpressions.

src/test/compile-fail/issue-22599.rs

@@ -0,0 +1,20 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![deny(unused_variables)]
+
+fn f(_: i32) {}
+
+fn main() {
+    let mut v = 0;
+    f(v);
+    v = match 0 { a => 0 }; //~ ERROR: unused variable: `a`
+    f(v);
+}