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

[Luke Lau via llvm-commits]

lukel97 wrote:

I've rebased and updated this to remove as many custom recipes and instructions as possible:

- The splat VPInstruction was removed and instead I reused the built in splatting mechanism in `VPTransformState::get`. I had to update it to handle splatting values that weren't instructions, and relax a `vputils::onlyFirstLaneUsed` assertion on VPScalarCastRecipe
- The VPStepVectorRecipe was removed and replaced with a VPInstruction that uses the same constant value trick in https://github.com/llvm/llvm-project/pull/119284 to specify the scalar type.

After https://github.com/llvm/llvm-project/pull/119284 lands I can rebase this on top of it to reuse it too. 

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