[llvm] [VPlan] Endow DerivedIV with IRFlags (PR #205908)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

I think I found one more case where we cannot preserve NUW when the IV counts down: https://alive2.llvm.org/ce/z/dRwiJb

https://github.com/llvm/llvm-project/pull/205908