[turbofan] Fix typing rule for number addition.

src/compiler/operation-typer.cc

 // 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.
 
 #include "src/compiler/operation-typer.h"
 
 #include "src/factory.h"
 #include "src/isolate.h"
 #include "src/type-cache.h"
 #include "src/types.h"
@@ skipping 281 matching lines @@
   if (!lhs->IsInhabited() || !rhs->IsInhabited()) {
     return Type::None();
   }
 
   // We can give more precise types for integers.
   if (!lhs->Is(cache_.kIntegerOrMinusZeroOrNaN) ||
       !rhs->Is(cache_.kIntegerOrMinusZeroOrNaN)) {
     return Type::Number();
   }
   Type* int_lhs = Type::Intersect(lhs, cache_.kInteger, zone());
+  if (lhs->Maybe(Type::MinusZero())) {
+    int_lhs = Type::Union(int_lhs, cache_.kSingletonZero, zone());
+  }
   Type* int_rhs = Type::Intersect(rhs, cache_.kInteger, zone());
+  if (rhs->Maybe(Type::MinusZero())) {
+    int_rhs = Type::Union(int_rhs, cache_.kSingletonZero, zone());
+  }
   Type* result =
       AddRanger(int_lhs->Min(), int_lhs->Max(), int_rhs->Min(), int_rhs->Max());
   if (lhs->Maybe(Type::NaN()) || rhs->Maybe(Type::NaN())) {
     result = Type::Union(result, Type::NaN(), zone());
   }
   if (lhs->Maybe(Type::MinusZero()) && rhs->Maybe(Type::MinusZero())) {
     result = Type::Union(result, Type::MinusZero(), zone());
   }
   return result;
 }
@@ skipping 157 matching lines @@
   return NumberAdd(lhs, rhs);
 }
 
 Type* OperationTyper::TypeJSSubtract(Type* lhs, Type* rhs) {
   return NumberSubtract(ToNumber(lhs), ToNumber(rhs));
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8