[mips] Fix using signaling NaN for holes in fixed double arrays.

test/mjsunit/regress/regress-crbug-620650.js

+// Copyright 2016 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.
+
+(function() {
+  function f(src, dst, i) {
+    dst[i] = src[i];
+  }
+  var buf = new ArrayBuffer(16);
+  var view_int32 = new Int32Array(buf);
+  view_int32[1] = 0xFFF7FFFF;
+  var view_f64 = new Float64Array(buf);
+  var arr = [,0.1];
+  f(view_f64, arr, -1);
+  f(view_f64, arr, 0);
+})();