[llvm] [SLP] More OOP to simplify vectorizeStores() (NFC) (PR #134605)

[Gaëtan Bossu via llvm-commits]

gbossu wrote:

Thanks for the initial review @alexey-bataev. Let me know if the refactoring itself sounds good to you, otherwise I'll fix the style issues and update the PR 🙂 

https://github.com/llvm/llvm-project/pull/134605