A JSArray may have a filler map in the elements pointer.

test/mjsunit/regress/regress-crbug-347903.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 --use-allocation-folding --verify-heap
+
+function f() {
+  var a = new Array(84632);
+  // Allocation folding will bail out trying to fold the elements alloc of
+  // array "b."
+  var b = new Array(84632);
+  var c = new Array(84632);
+  return [a, b, c];
+}
+f(); f();
+%OptimizeFunctionOnNextCall(f);
+for(var i = 0; i < 10; i++) {
+  f();
+}