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

[Luke Lau via llvm-commits]

lukel97 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?

I think it's fine if the operands aren't single scalar, during `VPDerivedIVRecipe::execute` it will just extract the first lane

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