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

[Florian Hahn via llvm-commits]

fhahn wrote:

> Why we cannot create VPScalarPHIRecipe immediately instead of using it to replace some other (abstract) recipes?

For VPCanonicalIVPHIRecipe I think the benefit from retaining a dedicated recipe is that multiple places use and rely on properties of the canonical IV (and do transforms based on it) and a dedicated recipe makes it trivial to identify and enforce it.

For the EVL-based PHI, I don;t think that is the case at the moment, so this could directly be a VPScalarPHIRecipe. It may be useful to retain the dedicated recipe though, as in the future a possible transform for the `prepareToCodegen` phase may be replacing the canonical IV with the EVL based phi altogether (something I recently talked about with @preames )

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