[turbofan] Add constant-folding for Float64RoundDown.

src/compiler/machine-operator-reducer.h

Index: src/compiler/machine-operator-reducer.h
diff --git a/src/compiler/machine-operator-reducer.h b/src/compiler/machine-operator-reducer.h
index d0845d9fabd01a061f099ad6051edd8a6c3b6ec2..164f9b84bd0cef9080733dcc9b6546b4ceb45c16 100644
--- a/src/compiler/machine-operator-reducer.h
+++ b/src/compiler/machine-operator-reducer.h
@@ -96,6 +96,7 @@ class V8_EXPORT_PRIVATE MachineOperatorReducer final
   Reduction ReduceFloat64InsertLowWord32(Node* node);
   Reduction ReduceFloat64InsertHighWord32(Node* node);
   Reduction ReduceFloat64Compare(Node* node);
+  Reduction ReduceFloat64RoundDown(Node* node);
 
   Graph* graph() const;
   JSGraph* jsgraph() const { return jsgraph_; }