Use signaling NaN for holes in fixed double arrays.

test/mjsunit/regress/regress-undefined-nan.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 loader(dst, src, i) {
+  dst[i] = src[i];
+}
+
+var ab = new ArrayBuffer(8);
+var i_view = new Int32Array(ab);
+i_view[0] = 0x7FF00000;
+i_view[1] = 0x7FF00000;
+var f_view = new Float64Array(ab);
+
+var fixed_double_elements = new Float64Array(1);
+fixed_double_elements[0] = f_view[0];
+
+var doubles = [0.5];
+loader(doubles, fixed_double_elements, 0);
+loader(doubles, fixed_double_elements, 0);
+%OptimizeFunctionOnNextCall(loader);
+loader(doubles, fixed_double_elements, 0);
+assertTrue(doubles[0] !== undefined);