[turbofan] Add constant-folding for Float64RoundDown.

src/compiler/machine-operator-reducer.cc

Index: src/compiler/machine-operator-reducer.cc
diff --git a/src/compiler/machine-operator-reducer.cc b/src/compiler/machine-operator-reducer.cc
index 8ee4224b13fb048f03cceabe4c62ee69ba094cdc..cc952ecc49bc5fc4476666ba72c8a6e83d945f79 100644
--- a/src/compiler/machine-operator-reducer.cc
+++ b/src/compiler/machine-operator-reducer.cc
@@ -664,6 +664,8 @@ Reduction MachineOperatorReducer::Reduce(Node* node) {
     case IrOpcode::kFloat64LessThan:
     case IrOpcode::kFloat64LessThanOrEqual:
       return ReduceFloat64Compare(node);
+    case IrOpcode::kFloat64RoundDown:
+      return ReduceFloat64RoundDown(node);
     default:
       break;
   }
@@ -1391,6 +1393,14 @@ Reduction MachineOperatorReducer::ReduceFloat64Compare(Node* node) {
   return NoChange();
 }
 
+Reduction MachineOperatorReducer::ReduceFloat64RoundDown(Node* node) {
+  DCHECK_EQ(IrOpcode::kFloat64RoundDown, node->opcode());
+  Float64Matcher m(node->InputAt(0));
+  if (m.HasValue()) {
+    return ReplaceFloat64(Floor(m.Value()));
+  }
+  return NoChange();
+}
 
 CommonOperatorBuilder* MachineOperatorReducer::common() const {
   return jsgraph()->common();