[llvm] [VPlan] Iterate over header phis to determine FORs that need EVL fixup. NFCI (PR #147032)

[Luke Lau via llvm-commits]

lukel97 wrote:

> LGTM, but `transformRecipestoEVLRecipes` is getting large — would it make sense to extract this part into a static helper function for better readability?

Agreed, I can look into a separate PR to move that into its own function

https://github.com/llvm/llvm-project/pull/147032