Don't propagate information through phis in loop headers.

test/mjsunit/regress/regress-check-eliminate-loop-phis.js

+// Copyright 2014 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 f() {
+  var o = {x:1};
+  var y = {y:2.5, x:0};
+  var result;
+  for (var i = 0; i < 2; i++) {
+    result = o.x + 3;
+    o = y;
+  }
+  return result;
+}
+
+f();
+f();
+%OptimizeFunctionOnNextCall(f);
+assertEquals(3, f());