Ignore never-initialized locals for unused_mut.

This commit filters out locals that have never been initialized for
consideration in the `unused_mut` lint.

This is intended to detect when the statement that would have
initialized the local was removed as unreachable code. In these cases,
we would not want to lint. This is the same behaviour as the AST borrow
checker.

This is achieved by taking advantage of an existing pass over the MIR
for the `unused_mut` lint and creating a set of those locals that were
never initialized.

src/test/ui/nll/issue-55344.rs

@@ -0,0 +1,26 @@
+// Copyright 2017 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.
+
+// compile-pass
+
+#![feature(nll)]
+#![allow(unreachable_code)]
+#![deny(unused_mut)]
+
+pub fn foo() {
+    return;
+
+    let mut v = 0;
+    assert_eq!(v, 0);
+    v = 1;
+    assert_eq!(v, 1);
+}
+
+fn main() {}