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

src/compiler/representation-change.cc

 // 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/compiler/representation-change.h"
 
 #include <sstream>
 
 #include "src/base/bits.h"
 #include "src/code-factory.h"
@@ skipping 609 matching lines @@
     IrOpcode::Value opcode) {
   switch (opcode) {
     case IrOpcode::kSpeculativeNumberAdd:
       return simplified()->CheckedInt32Add();
     case IrOpcode::kSpeculativeNumberSubtract:
       return simplified()->CheckedInt32Sub();
     case IrOpcode::kSpeculativeNumberDivide:
       return simplified()->CheckedInt32Div();
     case IrOpcode::kSpeculativeNumberModulus:
       return simplified()->CheckedInt32Mod();
-    case IrOpcode::kSpeculativeNumberMultiply:
-      return simplified()->CheckedInt32Mul();
     default:
       UNREACHABLE();
       return nullptr;
   }
 }
 
 const Operator* RepresentationChanger::Uint32OperatorFor(
     IrOpcode::Value opcode) {
   switch (opcode) {
     case IrOpcode::kNumberAdd:
@@ skipping 167 matching lines @@
 }
 
 Node* RepresentationChanger::InsertChangeTaggedToFloat64(Node* node) {
   return jsgraph()->graph()->NewNode(simplified()->ChangeTaggedToFloat64(),
                                      node);
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8