[llvm] [VPlan] Replicate VPScalarIVStepsRecipe by VF outside replicate regions. (PR #170053)

[via llvm-commits]

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ayalz wrote:

Independent thoughts:

VPScalarIVStepsRecipe works for scalable VF too, provided only first lane is used and UF>1? Worth clarifying/documenting.
For fixed VF, could StartIndex be computed using APInt methods instead of relying on VPBuilder foldings below?

https://github.com/llvm/llvm-project/pull/170053