[llvm] [VPlan] Move widening decisions to VPlanWideningDecisions.cpp (NFC) (PR #209887)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM, I think the most important thing here is to trim down VPlanTransforms. "Widening" to me kind of means any decision to change the width of a recipe, either making it narrower or wider. 

https://github.com/llvm/llvm-project/pull/209887