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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> Maybe worth checking if this has any compile time impact?

I've created https://github.com/RKSimon/llvm-project/tree/perf/pr66991 - it should appear on https://llvm-compile-time-tracker.com shortly

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