[Turbofan]: Eliminate the check for -0 if it's not possible/observable.

test/mjsunit/compiler/math-mul.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.
 
 // Flags: --allow-natives-syntax
 
+// For TurboFan, make sure we can eliminate the -0 return value check
+// by recognizing a constant value.
+function gotaconstant(y) { return 15 * y; }
+assertEquals(45, gotaconstant(3));
+gotaconstant(3);
+%OptimizeFunctionOnNextCall(gotaconstant);
+gotaconstant(3);
+
+function gotaconstant_truncated(x, y) { return x * y | 0; }
+assertEquals(45, gotaconstant_truncated(3, 15));
+gotaconstant_truncated(3, 15);
+%OptimizeFunctionOnNextCall(gotaconstant_truncated);
+gotaconstant_truncated(3, 15);
+
 function test(x, y) { return x * y; }
 
 assertEquals(12, test(3, 4));
 assertEquals(16, test(4, 4));
 
 %OptimizeFunctionOnNextCall(test);
 assertEquals(27, test(9, 3));
 
 assertEquals(-0, test(-3, 0));
 assertEquals(-0, test(0, -0));
 
 
 const SMI_MAX = (1 << 29) - 1 + (1 << 29);  // Create without overflowing.
 const SMI_MIN = -SMI_MAX - 1;  // Create without overflowing.
 
 // multiply by 3 to avoid compiler optimizations that convert 2*x to x + x.
 assertEquals(SMI_MAX + SMI_MAX + SMI_MAX, test(SMI_MAX, 3));