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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Hmm - doesn't look great https://llvm-compile-time-tracker.com/compare.php?from=66e1d2c96a194f572be5b373705f493b1a4dc811&to=29fbcd00a16a199cd94533dac76b20ba069527b4&stat=instructions:u

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