[llvm] [DA] remove getSplitIteration (PR #167698)

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj commented:

I think `DVEntry::Splitable` and `Dependnece::isSplitable` no longer make much sense. Maybe they should be removed as well. Otherwise LGTM.

https://github.com/llvm/llvm-project/pull/167698