[llvm] VPlan: prepare to recursively simplify recipes (NFC) (PR #105699)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

I investigated this thoroughly, and it is impossible to further simplify a `VPWidenCastRecipe`. I've now turned the patch into a pure NFC change, assuming this is what @fhahn meant, and will update the real patch shortly.

https://github.com/llvm/llvm-project/pull/105699