When splitting a block during a dialect conversion, a
`SplitBlockRewrite` object is stored in the dialect conversion state.
This commit removes `SplitBlockRewrite`. Instead, a combination of
`CreateBlockRewrite` and multiple `MoveOperationRewrite` is used.
This change simplifies the internal state of the dialect conversion and
is also needed to properly support listeners.
`RewriteBase::splitBlock` is now no longer virtual. All necessary
information for committing/rolling back a split block rewrite can be
deduced from `Listener::notifyBlockInserted` and
`Listener::notifyOperationInserted` (which is also called when moving an
operation).
926a19bf0b