Slightly simplify Math.sign and Math.trunc.

src/math.js

 // Copyright 2012 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.
 
 "use strict";
 
 // This file relies on the fact that the following declarations have been made
 // in runtime.js:
 // var $Object = global.Object;
 
@@ skipping 155 matching lines @@
 // Non-standard extension.
 function MathImul(x, y) {
   return %NumberImul(TO_NUMBER_INLINE(x), TO_NUMBER_INLINE(y));
 }
 
 // ES6 draft 09-27-13, section 20.2.2.28.
 function MathSign(x) {
   x = TO_NUMBER_INLINE(x);
   if (x > 0) return 1;
   if (x < 0) return -1;
-  if (x === 0) return x;
-  return NAN;
+  // -0, 0 or NaN.
+  return x;
 }
 
 // ES6 draft 09-27-13, section 20.2.2.34.
 function MathTrunc(x) {
   x = TO_NUMBER_INLINE(x);
   if (x > 0) return MathFloor(x);
   if (x < 0) return MathCeil(x);
-  if (x === 0) return x;
-  return NAN;
+  // -0, 0 or NaN.
+  return x;
 }
 
 // ES6 draft 09-27-13, section 20.2.2.30.
 function MathSinh(x) {
   if (!IS_NUMBER(x)) x = NonNumberToNumber(x);
   // Idempotent for NaN, +/-0 and +/-Infinity.
   if (x === 0 || !NUMBER_IS_FINITE(x)) return x;
   return (MathExp(x) - MathExp(-x)) / 2;
 }
 
@@ skipping 195 matching lines @@
     "expm1", MathExpm1    // implemented by third_party/fdlibm
   ));
 
   %SetInlineBuiltinFlag(MathCeil);
   %SetInlineBuiltinFlag(MathRandom);
   %SetInlineBuiltinFlag(MathSin);
   %SetInlineBuiltinFlag(MathCos);
 }
 
 SetUpMath();