[llvm] [LV] Move some if-reduction.ll tests to iv-select-cmp.ll (PR #141769)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

might be better to pull out the find-first-iv tests into a separate file, rather than further growing iv-select-cmp?

https://github.com/llvm/llvm-project/pull/141769