[clang] [mlir][llvm] Fix elem type passing into `getelementptr` (PR #68136)

[Rik Huijzer via cfe-commits]

rikhuijzer wrote:

> [...] I am a bit afraid of the burden of having to maintain what is essentially a third GEP representation.

Thanks for the explanation @zero9178. I am affraid I do not fully understand your reasoning (most likely due to a lack of knowledge on my side). However, it sounds very reasonable to me!

So before I spend more time on the code, shall I rewrite this PR to throw a clear error from `GEPOp::verify` for the incorrect representation? Then we can close https://github.com/llvm/llvm-project/issues/63832.

https://github.com/llvm/llvm-project/pull/68136