[turbofan] Allow OptionalOperator to return a placeholder.

test/unittests/compiler/int64-lowering-unittest.cc

Index: test/unittests/compiler/int64-lowering-unittest.cc
diff --git a/test/unittests/compiler/int64-lowering-unittest.cc b/test/unittests/compiler/int64-lowering-unittest.cc
index 804c399b40036f8473da61e41364d5fa7832b476..934756bf600aaf4569c6c2f84e7827dcf34ebf8a 100644
--- a/test/unittests/compiler/int64-lowering-unittest.cc
+++ b/test/unittests/compiler/int64-lowering-unittest.cc
@@ -628,7 +628,7 @@ TEST_F(Int64LoweringTest, I64Clz) {
 }
 
 TEST_F(Int64LoweringTest, I64Ctz) {
-  LowerGraph(graph()->NewNode(machine()->Word64CtzPlaceholder(),
+  LowerGraph(graph()->NewNode(machine()->Word64Ctz().placeholder(),
                               Int64Constant(value(0))),
              MachineRepresentation::kWord64);
   Capture<Node*> branch_capture;
@@ -668,7 +668,7 @@ TEST_F(Int64LoweringTest, Dfs) {
 }
 
 TEST_F(Int64LoweringTest, I64Popcnt) {
-  LowerGraph(graph()->NewNode(machine()->Word64PopcntPlaceholder(),
+  LowerGraph(graph()->NewNode(machine()->Word64Popcnt().placeholder(),
                               Int64Constant(value(0))),
              MachineRepresentation::kWord64);