[turbofan] Add constant-folding for Float64RoundDown.

test/unittests/compiler/machine-operator-reducer-unittest.cc

Index: test/unittests/compiler/machine-operator-reducer-unittest.cc
diff --git a/test/unittests/compiler/machine-operator-reducer-unittest.cc b/test/unittests/compiler/machine-operator-reducer-unittest.cc
index 474a27038a749bae3e5eb0eae4a4d01c4c852450..4f1946c379426897d9920c252dd4e2592fc53c7d 100644
--- a/test/unittests/compiler/machine-operator-reducer-unittest.cc
+++ b/test/unittests/compiler/machine-operator-reducer-unittest.cc
@@ -2085,8 +2085,19 @@ TEST_F(MachineOperatorReducerTest, Float64LessThanOrEqualWithFloat32Constant) {
 
 
 // -----------------------------------------------------------------------------
-// Store
+// Float64RoundDown
 
+TEST_F(MachineOperatorReducerTest, Float64RoundDownWithConstant) {
+  TRACED_FOREACH(double, x, kFloat64Values) {
+    Reduction r = Reduce(graph()->NewNode(
+        machine()->Float64RoundDown().placeholder(), Float64Constant(x)));
+    ASSERT_TRUE(r.Changed());
+    EXPECT_THAT(r.replacement(), IsFloat64Constant(Floor(x)));
+  }
+}
+
+// -----------------------------------------------------------------------------
+// Store
 
 TEST_F(MachineOperatorReducerTest, StoreRepWord8WithWord32And) {
   const StoreRepresentation rep(MachineRepresentation::kWord8, kNoWriteBarrier);