[mlir][IR] Change `notifyBlockCreated` to `notifyBlockInserted` (#79472)

This change makes the callback consistent with
`notifyOperationInserted`: both now notify about IR insertion, not IR
creation. See also #78988.

This change also simplifies the dialect conversion: it is no longer
necessary to override the `inlineRegionBefore` method. All information
that is necessary for rollback is provided with the
`notifyBlockInserted` callback.