[llvm] [SLP] Use named structs in vectorizeStores() (NFC) (PR #132781)

[Gaëtan Bossu via llvm-commits]

gbossu wrote:

FYI @alexey-bataev , let me know if you're fine with this small change. If so, I wouldn't mind pushing other NFC patches as I browse through the code 🙂   

https://github.com/llvm/llvm-project/pull/132781