[turbofan] Improve representation selection for type guard.

test/mjsunit/compiler/regress-726554.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 h(a,b){
+  for(var i=0; i<a.length; i++) {h(a[i],b[i]); }
+}
+
+function g() {
+  h(arguments.length, 2);
+}
+
+function f() {
+  return g(1, 2);
+}
+
+b = [1,,];
+b[1] = 3.5;
+
+h(b, [1073741823, 2147483648, -12]);
+
+f();
+f();
+%OptimizeFunctionOnNextCall(f);
+f();