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

[Gaëtan Bossu via llvm-commits]

gbossu wrote:

I'd be happy if we could push this small change upstream @alexey-bataev , I think it's an incremental improvement for readability. What do you think?

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