[llvm] [VPlan] Add opcode to create step for wide inductions. (PR #119284)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> Do we need to return true for WideIVStep in VPInstruction::onlyFirstLaneUsed if both inputs are scalar?

I haven't checked if there are any such cases, are there by the uses in the follow-up PR?

https://github.com/llvm/llvm-project/pull/119284