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

[Luke Lau via llvm-commits]

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lukel97 wrote:

Woops, will remove. This sometimes gets left around when calling update_test_checks.py on files that check the stderr output, e.g. `; RUN: opt -S < %s -p loop-vectorize -debug-only=loop-vectorize -mattr=+sve 2>%t`. Not sure if UTC could be taught to redirect that

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