[llvm] [IR] Fix assertion error in User new/delete edge case (PR #129914)

[Marc Auberer via llvm-commits]

marcauberer wrote:

@nikic Probably you are right, I am not particularly familiar in this corner ;)
Do you suggest to revert this and do it properly or live with this quick fix for now and apply the proper solution on top of it?

https://github.com/llvm/llvm-project/pull/129914