[llvm] [VPlan] Add VPPhiAccessors to provide interface for phi recipes (NFC) (PR #138472)

[Luke Lau via llvm-commits]

https://github.com/lukel97 commented:

Does the PR title need updated? VPPhiAccessors was already added in #129388, this is adding more methods to it right?

https://github.com/llvm/llvm-project/pull/138472