[turbofan] Add constant-folding for Float64RoundDown.

test/unittests/compiler/machine-operator-reducer-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-reducer.h"
 #include "src/base/bits.h"
 #include "src/base/division-by-constant.h"
 #include "src/base/ieee754.h"
 #include "src/compiler/js-graph.h"
 #include "src/compiler/typer.h"
@@ skipping 2067 matching lines @@
           graph()->NewNode(machine()->ChangeFloat32ToFloat64(), p0)));
       ASSERT_TRUE(r.Changed());
       EXPECT_THAT(r.replacement(),
                   IsFloat32LessThanOrEqual(IsFloat32Constant(x), p0));
     }
   }
 }
 
 
 // -----------------------------------------------------------------------------
+// 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);
   Node* const base = Parameter(0);
   Node* const index = Parameter(1);
   Node* const value = Parameter(2);
   Node* const effect = graph()->start();
   Node* const control = graph()->start();
   TRACED_FOREACH(uint32_t, x, kUint32Values) {
     Node* const node =
         graph()->NewNode(machine()->Store(rep), base, index,
@@ skipping 76 matching lines @@
     Reduction r = Reduce(node);
     ASSERT_TRUE(r.Changed());
     EXPECT_THAT(r.replacement(),
                 IsStore(rep, base, index, value, effect, control));
   }
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8