Fix (optimizer) crashes in conditional nodes when result is not needed.

tests/language/nullaware_opt_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 //
 // SharedOptions=--enable-null-aware-operators
 // VMOptions=--optimization_counter_threshold=5
 //
 // Basic null-aware operator test that invokes the optimizing compiler.
 
 import "package:expect/expect.dart";
@@ skipping 39 matching lines @@
 
   c?.f ??= 400;
   Expect.equals(null, c?.f);
   Expect.equals(null, c?.f++);
   e?.f++;
   Expect.equals(201, e.f);
 
   var x = 5 ?? bomb();
 }
 
+// Check that instructions without result do not crash.
+test2() {
+  var c;
+  c?.v;
+  c?.m(bomb());
+}
+
 main() {
   for (int i = 0; i < 10; i++) {
     test();
+    test2();
   }
 }