[llvm] [SDag] Notify listeners when deleting a node (PR #66991)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Every time we encounter this kind of situation I come back to fixing topological ordering in DAG once and for all.....

I am a big fan of [D152928](https://reviews.llvm.org/D152928) but would it completely eliminate this deleted-nodes bug?

https://github.com/llvm/llvm-project/pull/66991