[llvm] [InstSimplify] Fold `getelementptr inbounds null, idx -> null` (PR #130742)

[Nikita Popov via llvm-commits]

nikic wrote:

I'm ok with reverting this until the `__PTR_ALIGN` workarounds in clang have landed. We should reapply directly after that though, to ensure more of these issues get caught as soon as possible.

https://github.com/llvm/llvm-project/pull/130742