It should not be possible to construct one without a triple. It would
also be nice to delete TargetLibraryInfoWrapperPass, but that is more
difficult.

This patch implements dependency maintenance upon receiveing the
notification that an instruction gets deleted.

This patch adds Memory successors to the memory nodes of the DAG. This
will help maintain the memory dependencies when nodes get removed.

This patch implements automatic DAG updating whenever a Use is set. This
maintains the UnscheduledSuccs counter that the scheduler relies on.

This patch fixes a bug in `DependencyGraph::extend()` when the old
interval contains no memory instructions. When this is the case we
should do a full dependency scan of the new interval.

The DAG will now receive a callback whenever a new instruction is
created and will update itself accordingly.

This patch fixes a bug in the dependency node iterators that would
incorrectly not skip nodes that are not in the current DAG. This
resulted in iterators returning nullptr when dereferenced.

The fix is to update the existing "skip" function to not only skip
non-instruction values but also to skip instructions not in the DAG.

This patch fixes a bug in the maintenance of the MemDGNode chain of the
DAG. Whenever we move a memory instruction, the DAG gets notified about
the move and maintains the chain of memory nodes. The bug was that if
the destination of the move was not a memory instruction, then the
memory node's next node would end up pointing to itself.

This patch implements the move instruction notifier for the DAG.
Whenever an instruction moves the notifier will maintain the DAG.