[llvm] [IR] Fix User use-after-destroy by zapping in ~User (PR #170575)

[Nikita Popov via llvm-commits]

nikic wrote:

compile-time: https://llvm-compile-time-tracker.com/compare.php?from=e66e8aac6e4c0cc3874cabad736233440b0ffb1e&to=3e1ceea55bb32ec5630892a0017e105663a31640&stat=instructions:u
max-rss: https://llvm-compile-time-tracker.com/compare.php?from=e66e8aac6e4c0cc3874cabad736233440b0ffb1e&to=3e1ceea55bb32ec5630892a0017e105663a31640&stat=max-rss

Looks like the compile-time impact is barely above the noise floor, and max-rss is in the noise (though I should say that the noise for that one is high, so mostly this tells us that there is no *major* memory usage regression).

https://github.com/llvm/llvm-project/pull/170575