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

[Markus Böck via cfe-commits]

zero9178 wrote:

 > 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 #63832.

I'd personally be in favour of that. There is a **lot** of room for improvements in the verifiers in the LLVM Dialect so any contributions there are definitely highly appreciated! 
That said, its probably best to wait a bit for others to comment as well as they may have different views than I do.

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