[turbofan] Mark operator constructors as const in MachineOperatorBuilder.

src/compiler/machine-operator-reducer-unittest.cc

 // Copyright 2014 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/base/bits.h"
 #include "src/compiler/graph-unittest.h"
 #include "src/compiler/js-graph.h"
 #include "src/compiler/machine-operator-reducer.h"
 #include "src/compiler/typer.h"
 
@@ skipping 146 matching lines @@
 }  // namespace
 
 
 // -----------------------------------------------------------------------------
 // Unary operators
 
 
 namespace {
 
 struct UnaryOperator {
-  const Operator* (MachineOperatorBuilder::*constructor)();
+  const Operator* (MachineOperatorBuilder::*constructor)() const;
   const char* constructor_name;
 };
 
 
 std::ostream& operator<<(std::ostream& os, const UnaryOperator& unop) {
   return os << unop.constructor_name;
 }
 
 
 static const UnaryOperator kUnaryOperators[] = {
@@ skipping 352 matching lines @@
       r = Reduce(graph()->NewNode(common()->Projection(0), add));
       ASSERT_TRUE(r.Changed());
       EXPECT_THAT(r.replacement(), IsInt32Constant(z));
     }
   }
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8