[turbofan] Add constant-folding for Float64RoundDown.

src/compiler/machine-operator-reducer.h

 // 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.
 
 #ifndef V8_COMPILER_MACHINE_OPERATOR_REDUCER_H_
 #define V8_COMPILER_MACHINE_OPERATOR_REDUCER_H_
 
 #include "src/base/compiler-specific.h"
 #include "src/compiler/graph-reducer.h"
 #include "src/compiler/machine-operator.h"
@@ skipping 78 matching lines @@
   Reduction ReduceWord64Shr(Node* node);
   Reduction ReduceWord32Sar(Node* node);
   Reduction ReduceWord64Sar(Node* node);
   Reduction ReduceWord32And(Node* node);
   Reduction TryMatchWord32Ror(Node* node);
   Reduction ReduceWord32Or(Node* node);
   Reduction ReduceWord32Xor(Node* node);
   Reduction ReduceFloat64InsertLowWord32(Node* node);
   Reduction ReduceFloat64InsertHighWord32(Node* node);
   Reduction ReduceFloat64Compare(Node* node);
+  Reduction ReduceFloat64RoundDown(Node* node);
 
   Graph* graph() const;
   JSGraph* jsgraph() const { return jsgraph_; }
   CommonOperatorBuilder* common() const;
   MachineOperatorBuilder* machine() const;
 
   JSGraph* jsgraph_;
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_MACHINE_OPERATOR_REDUCER_H_