[turbofan] Make Factory::NewNumber() always return the minus_zero_value.

TEST=unittests
R=mstarzinger@chromium.org, dcarney@chromium.org

Review URL: https://codereview.chromium.org/838263006

Cr-Commit-Position: refs/heads/master@{#26080}

src/factory.cc

@@ -1018,7 +1018,7 @@ Handle<Object> Factory::NewNumber(double value,
   // We need to distinguish the minus zero value and this cannot be
   // done after conversion to int. Doing this by comparing bit
   // patterns is faster than using fpclassify() et al.
-  if (IsMinusZero(value)) return NewHeapNumber(-0.0, IMMUTABLE, pretenure);
+  if (IsMinusZero(value)) return minus_zero_value();
 
   int int_value = FastD2IChecked(value);
   if (value == int_value && Smi::IsValid(int_value)) {

test/unittests/factory-unittest.cc

@@ -0,0 +1,21 @@
+// Copyright 2015 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.
+
+#include "src/factory.h"
+#include "src/handles-inl.h"
+#include "test/unittests/test-utils.h"
+
+namespace v8 {
+namespace internal {
+
+typedef TestWithIsolate FactoryTest;
+
+
+TEST_F(FactoryTest, NewNumberWithMinusZero) {
+  Handle<Object> minus_zero_value = factory()->minus_zero_value();
+  EXPECT_TRUE(minus_zero_value.is_identical_to(factory()->NewNumber(-0.0)));
+}
+
+}  // namespace internal
+}  // namespace v8

test/unittests/unittests.gyp

@@ -67,6 +67,7 @@
         'compiler/simplified-operator-unittest.cc',
         'compiler/value-numbering-reducer-unittest.cc',
         'compiler/zone-pool-unittest.cc',
+        'factory-unittest.cc',
         'libplatform/default-platform-unittest.cc',
         'libplatform/task-queue-unittest.cc',
         'libplatform/worker-thread-unittest.cc',