[llvm] [IR] Consolidate OneNthElements IIT descriptors. NFCI (PR #141492)

[Nikita Popov via llvm-commits]

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

Last commit LGTM. The foreach + listsplat makes things nicer than I expected :)

https://github.com/llvm/llvm-project/pull/141492