[llvm] [VPlan] Scalarize to first-lane-only directly on VPlan (PR #184267)

[Ramkumar Ramachandra via llvm-commits]

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@@ -485,6 +485,12 @@ struct VPlanTransforms {
                                          VPRecipeBuilder &RecipeBuilder,
                                          VPCostContext &CostCtx,
                                          LoopVectorizationLegality &Legal);
+
+  /// Make VPlan-based scalarization decision prior to delegating to the ones
+  /// made by the legacy CM. Only transforms "usesFirstLaneOnly` def-use chains
+  /// enabled by prior widening of consecutive memory operations for now.
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artagnon wrote:

"the VPlan itself has no information about interleave groups at that point, so this pass has no way to figure out that this particular address computation is only used at first lane for operations belonging to an interleave group" -- I think you can replace your explanation about asserts in the PR description with this one, as I think it's more enlightening?

https://github.com/llvm/llvm-project/pull/184267