"const" is extraneous in the presence of "constexpr" for simple
variables and arrays.

When assigning a bundle to a DAG Node that is already assigned to a
SchedBundle we need to remove the node from the old bundle.

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

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 TransactionAcceptOrRevert pass is the final pass in the Sandbox
Vectorizer's default pass pipeline. It's job is to check the cost
before/after vectorization and accept or revert the IR to its original
state.

Since we are now starting the transaction in BottomUpVec, tests that run
a custom pipeline need to accept the transaction. This is done with the
help of the TransactionAlwaysAccept pass (tr-accept).

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 moves the check of whether a node exists before the check of
whether it is contained in the interval.

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.