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

[Nikita Popov via llvm-commits]

nikic wrote:

> > If this is causing a lot of issues can we possibly revert it while the fixes make their way through review, and then re-merge it with the fixes?
> 
> @efriedma-quic @nikic I am ok to just add a ubsan check in clang 21. Then we can reland this patch in llvm 22/23.

I'm probably missing something, but don't we already have a ubsan check for this for a long time? https://clang.godbolt.org/z/vGr7qYYjr

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