[llvm] [VPlan] Update getBestPlan to return VF, use also for epilogue vec. (PR #98821)

[Renato Golin via llvm-commits]

https://github.com/rengolin approved this pull request.

Looks like a simple cleanup, LGTM, but please wait a day in case others have comments.

https://github.com/llvm/llvm-project/pull/98821