[turbofan] Mark operator constructors as const in MachineOperatorBuilder.

src/compiler/machine-operator-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/compiler/machine-operator.h"
 #include "src/compiler/operator-properties-inl.h"
 #include "testing/gtest-support.h"
 
 namespace v8 {
 namespace internal {
@@ skipping 151 matching lines @@
                                              kFullWriteBarrier))));
 
 
 // -----------------------------------------------------------------------------
 // Pure operators.
 
 
 namespace {
 
 struct PureOperator {
-  const Operator* (MachineOperatorBuilder::*constructor)();
+  const Operator* (MachineOperatorBuilder::*constructor)() const;
   IrOpcode::Value opcode;
   int value_input_count;
   int value_output_count;
 };
 
 
 std::ostream& operator<<(std::ostream& os, const PureOperator& pop) {
   return os << IrOpcode::Mnemonic(pop.opcode);
 }
 
@@ skipping 132 matching lines @@
   EXPECT_EQ(machine.Int64UDiv(), machine.IntUDiv());
   EXPECT_EQ(machine.Int64Mod(), machine.IntMod());
   EXPECT_EQ(machine.Int64UMod(), machine.IntUMod());
   EXPECT_EQ(machine.Int64LessThan(), machine.IntLessThan());
   EXPECT_EQ(machine.Int64LessThanOrEqual(), machine.IntLessThanOrEqual());
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8