[turbofan] Don't mix element accesses with incompatible representations.

test/mjsunit/regress/regress-crbug-719479.js

+// Copyright 2017 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 baz(a, b) {
+  for (var i = 0; i < a.length; i++) {
+    if (a[i], b[i]) return false;
+  }
+}
+function bar(expected, found) {
+  if (!baz(found, expected)) {
+  }
+};
+bar([{}, 6, NaN], [1.8, , NaN]);
+function foo() {
+  var a = [1,2,3,4];
+  bar(a.length, a.length);
+}
+foo();
+foo();
+%OptimizeFunctionOnNextCall(foo);
+foo();