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

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> Yes, in the general case, add and mul need not necessarily have the same wrap flags. 

yeah I think it is a good first step to start with conservative flags. I think the latest version should be sound, just a few more suggestions inline

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