[llvm] [VPlan] Add VPInstruction::StepVector and use it in VPWidenIntOrFpInductionRecipe (PR #129508)

[Luke Lau via llvm-commits]

lukel97 wrote:

> It doesn't look the commit message now reflects the changes in the patch, since VPInstructionWithType seems to be already present in the code?

Oh I think it was trying to say that it added a new opcode for use with VPInstructionWithType, but I've reworded it to be more clear

https://github.com/llvm/llvm-project/pull/129508