Do not return the constant singleton float -0.0 from UnaryOpStub, because it ...

test/mjsunit/smi-negative-zero.js

 // Copyright 2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
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@@ skipping 80 matching lines @@
 assertEquals(Infinity, one / (0 * 0), "fisk4");
 assertEquals(1, one / (-1 * -1), "fisk5");
 
 assertEquals(-Infinity, one / (0 / -1), "hest1");
 assertEquals(Infinity, one / (0 / 1), "hest2");
 
 assertEquals(-Infinity, one / (-4 % 2), "fiskhest1");
 assertEquals(-Infinity, one / (-4 % -2), "fiskhest2");
 assertEquals(Infinity, one / (4 % 2), "fiskhest3");
 assertEquals(Infinity, one / (4 % -2), "fiskhest4");
+
+
+// This tests against a singleton -0.0 object being overwritten.
+x = 0;
+z = 3044;
+
+function foo(x) {
+  var y = -x + z;
+  return -x;
+}
+
+assertEquals(0, foo(x));
+assertEquals(0, foo(x));