[llvm] [VPlan] Mark DerivedIV unconditionally single-scalar (NFC) (PR #189706)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

Agreed that they are single scalar regardless of their operands. But I am surprised this is not causing any changes. I suppose in practice, the operands are always guaranteed to be single-scalar as well?

https://github.com/llvm/llvm-project/pull/189706