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

[Luke Lau via llvm-commits]

lukel97 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.

We could also get rid of the PHI recipe if we relax the native-path restriction on VPWidenPHIRecipe, see #118662 

There's some extra diff now due to how this doesn't emit an identity add when building the initial value, I've also opened a separate PR to split this off: #119668 



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