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

[Luke Lau via llvm-commits]

lukel97 wrote:

> 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?

That would be my guess too. I think the invariant that @eas is describing probably holds, we could assert that in a separate PR if we wanted

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