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

[Florian Hahn via llvm-commits]

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@@ -883,6 +883,8 @@ class VPInstruction : public VPRecipeWithIRFlags,
     AnyOf,
     // Calculates the first active lane index of the vector predicate operand.
     FirstActiveLane,
+    // Creates a step vector starting from 0 with a step of 1.
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fhahn wrote:

ah right, it needs to take the type. Would be convenient to have it as operand for some transforms (e.g. `narrowInterleaveGroups` but ok to leave it as-is for now.. Still good to document the range

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