[llvm] [SeparateConstOffsetFromGEP] Fix offsets across trunc and ext (PR #225195)

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

Thanks for pointing out #221381. I agree this is quite similar and generally a bit shorter/simpler. Since this version is approved and green, I plan to land it tomorrow unless @Peter9606 or the reviewers have any concerns?

https://github.com/llvm/llvm-project/pull/225195