[turbofan] Check for dead node in the common operator reducer.

test/mjsunit/regress/regress-556543.js

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --allow-natives-syntax
 
-var a = [];
-for (var i = -20; i < 0; ++i) {
-  a[i] = 0;
+function f() {
+  for (var __v_2 = 0; __v_2 < __v_5; ++__v_2) {
+    for (var __v_5 = 0; __v_3 < 1; ++__v_8) {
+      if (true || 0 > -6) continue;
+      for (var __v_3 = 0; __v_3 < 1; ++__v_3) {
+      }
+    }
+  }
 }
-
-function g() {
-    [].push.apply(a, arguments);
-}
-
-function f() {
-  g();
-}
-
-g();
-g();
 %OptimizeFunctionOnNextCall(f);
 f();