[llvm] [VPlan] Expand VPWidenIntOrFpInductionRecipe into separate recipes (PR #118638)

[Luke Lau via llvm-commits]

lukel97 wrote:

> For the induction step, there's already code the materializes it (for unrolling), #119284 generalizes it to a VPInstruction opcode. Would that help?

I think I can reuse that here, that would take care of the casting part. And I guess we would get the benefit of the constant step optimization too. I'll rebase this PR on top of it

I think we would need to expand the VPWidenIntOrFpInductionRecipes first in convertToConcreteRecipes so that the WideIVSteps get a chance to be expanded afterwards?

https://github.com/llvm/llvm-project/pull/118638