[llvm] Revert "[InstSimplify] Fold `getelementptr inbounds null, idx -> null` (#130742)" (PR #138168)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

I will be out of the office tomorrow (UTC+8). Feel free to merge this revert if the CI is green :)

I don't know what a suitable schedule is to reland https://github.com/llvm/llvm-project/pull/138168. @nikic [suggested](https://github.com/llvm/llvm-project/pull/130742#issuecomment-2844970193) to reland it after https://github.com/llvm/llvm-project/issues/137833 is resolved. I am fine to reland it after one or N releases since the ubsan checker is available.

The last such incident was the introduction of `getelementptr nusw/nuw`. Downstream users complain about "miscompilation" in Clang, even if the pointer overflow checker is available. It is a heavy burden for us compiler developers to explain what UB is to our users over and over again :(

Anyway, we need a policy to reserve enough time for downstream users to tackle potential breakages.


https://github.com/llvm/llvm-project/pull/138168