Add optimized helper for Mark/CheckProcessed

MarkStep uses logic like this in a few places:

```csharp
if (reason.Kind == DependencyKind.AlreadyMarked) {
Debug.Assert (Annotations.IsMarked (member));
} else {
Annotations.Mark (member, reason);
}
// possibly do extra work
if (CheckProcessed (member))
return;
// do main work for the member
```

The `AlreadyMarked` check is to avoid calling Annotations.Mark twice when a different step marks a member through Annotations, and the `CheckProcessed` call ensures that most of the work is done only once per marked member. Sometimes there is logic in-between which can get called once for each time the member is marked.

We could avoid the `AlreadyMarked` check by splitting up the MarkStep.Mark* methods into variants that do/don't call Annotations.Mark, or by moving the check into `Annotations.Mark`.

In cases where there is no "extra work" done between `Mark` and `CheckProcessed`, we could then combine the `Mark/CheckProcessed` calls (assuming it is ok to remove the `CheckProcessed` extensibility point for derived classes).
With the extra tracking introduced by https://github.com/mono/linker/pull/1768, this would avoid an extra hashset lookup in a few cases.

See https://github.com/mono/linker/pull/1768#discussion_r562740348