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

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

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

Thanks, I searched exhaustively; it looks like no flags are valid when counting down: https://alive2.llvm.org/ce/z/Znorf7

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