[turbofan] Ignore dead cached nodes in the JSGraph.

test/mjsunit/regress/regress-crbug-480807.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 --use-osr --turbo-osr --noalways-opt
+
+function foo() {
+  var c = 0;
+  for (var e = 0; e < 1; ++e) {
+    for (var a = 1; a > 0; a--) {
+      c += 1;
+    }
+    for (var b = 1; b > 0; b--) {
+      %OptimizeOsr();
+    }
+  }
+  return c;
+}
+try {
+  foo();
+} catch (e) {
+}