Fix ICE when returning a variable whose declaration is unreachable

Fixes #24353

src/librustc_trans/trans/controlflow.rs

@@ -131,7 +131,9 @@ pub fn trans_block<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
 
     match b.expr {
         Some(ref e) => {
-            bcx = expr::trans_into(bcx, &**e, dest);
+            if !bcx.unreachable.get() {
+                bcx = expr::trans_into(bcx, &**e, dest);
+            }
         }
         None => {
             assert!(dest == expr::Ignore || bcx.unreachable.get());

src/test/run-pass/issue24353.rs

@@ -0,0 +1,16 @@
+// 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.
+
+fn main() {
+    return ();
+
+    let x = ();
+    x
+}