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

[Nikita Popov via llvm-commits]

nikic wrote:

Compile-time: https://llvm-compile-time-tracker.com/compare.php?from=a614f2b489caa19001b2f44784514a6226f79cb7&to=e2454c7cb4245fceb05435cdcf775d4b1a47fc24&stat=instructions:u Impact is very small if there is any.

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