[turbofan] Make an OptionalOperator for MachineOperatorBuilder.

test/unittests/compiler/raw-machine-assembler.h

Index: test/unittests/compiler/raw-machine-assembler.h
diff --git a/test/unittests/compiler/raw-machine-assembler.h b/test/unittests/compiler/raw-machine-assembler.h
index d4aeab33c52efc927c64e862e2bfcd4f6f9d22c2..45e1e3ec31e27d00b25d61b6ce93d8b69eab89bf 100644
--- a/test/unittests/compiler/raw-machine-assembler.h
+++ b/test/unittests/compiler/raw-machine-assembler.h
@@ -416,13 +416,13 @@ class RawMachineAssembler : public GraphBuilder {
     return NewNode(machine()->TruncateInt64ToInt32(), a);
   }
   Node* Float64RoundDown(Node* a) {
-    return NewNode(machine()->Float64RoundDown(), a);
+    return NewNode(machine()->Float64RoundDown().op(), a);
   }
   Node* Float64RoundTruncate(Node* a) {
-    return NewNode(machine()->Float64RoundTruncate(), a);
+    return NewNode(machine()->Float64RoundTruncate().op(), a);
   }
   Node* Float64RoundTiesAway(Node* a) {
-    return NewNode(machine()->Float64RoundTiesAway(), a);
+    return NewNode(machine()->Float64RoundTiesAway().op(), a);
   }
 
   // Float64 bit operations.