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

[Florian Hahn via llvm-commits]

fhahn wrote:

> I've reworked this to remove the IV specific initial value and backedge recipes, and instead they're modelled in VPlan during expansion.
> 
> I had to add two new recipes to model splats and step vectors in VPlan. I needed the former because although it looks like you can broadcast underlying scalar IR values, I couldn't see a way to broadcast a scalar VPValue with no underlying instruction.
> 

For the induction step, there's already code the materializes it (for unrolling), https://github.com/llvm/llvm-project/pull/119284 generalizes it to a VPInstruction opcode. Would that help?

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