[llvm] Fix incorrect codegen with respect to GEPs #85333 (PR #88440)

[Paul Kirth via llvm-commits]

https://github.com/ilovepi approved this pull request.

Overall, this seems fine to me, but I do wonder about the situation arising in the first place. I was under the impression that GEPs had been canonicalized to always use i8 now when the IR is read in (e.g. auto upgrade). Is that logic in one of the simplification passes? Does that imply that such a pass should always run *before* **GVNSink**? IIRC, what I'm referring to is related to the [ptradd proposal](https://discourse.llvm.org/t/rfc-replacing-getelementptr-with-ptradd/68699). 


https://github.com/llvm/llvm-project/pull/88440