[turbofan] Fix variable liveness control structure creation.

test/mjsunit/compiler/regress-variable-liveness.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
+
+function foo(x) {
+  %DeoptimizeFunction(run);
+  return x;
+}
+
+function run() {
+  var line = new Array(2);
+  for (var n = 3; n > 0; n = n - 1) {
+    if (n < foo(line.length)) line = new Array(n);
+    line[0] = n;
+  }
+}
+
+assertEquals(void 0, run());
+%OptimizeFunctionOnNextCall(run);
+assertEquals(void 0, run());