[llvm] [VPlan] Introduce VPScalarPHIRecipe, use for can & EVL IV codegen (NFC). (PR #114305)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

Thanks @ayalz, first set of comments address in afef545, will follow up on the remainder soon

https://github.com/llvm/llvm-project/pull/114305