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

[Andrei Elovikov via llvm-commits]

eas wrote:

This ctor: https://github.com/llvm/llvm-project/blob/2537596d2292d32e029dbccf4ecee55202c8ec64/llvm/lib/Transforms/Vectorize/VPlan.h#L4020-L4024

makes it so that this property isn't really guaranteed by the construction. Maybe add an assert that all its operands **must** be single-scalar as well?

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