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

test/mjsunit/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
-
-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));