[mlir][IR] Do not trigger `notifyOperationInserted` for unlinked ops (#80278)

This commit changes `OpBuilder::create` and `OpBuilder::createOrFold`
such that `notifyOperationInserted` is no longer triggered if no
insertion point is set. In such a case, an unlinked operation is created
but not inserted, so `notifyOperationInserted` should not be triggered.

Note: Inserting another op into a block that belongs to an unlinked op
(e.g., by the builder of the unlinked op) will trigger a notification.