[llvm] [VPlan] First step towards VPlan cost modeling. (PR #67934)

[via llvm-commits]

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@@ -361,6 +364,9 @@ class LoopVectorizationPlanner {
   /// Return the best VPlan for \p VF.
   VPlan &getBestPlanFor(ElementCount VF) const;
 
+  /// Return the most profitable plan.
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ayalz wrote:

nit: every plan contains its VF range; reduce the range of the best plan to a single value, instead of passing it alongside? Method should be const?

https://github.com/llvm/llvm-project/pull/67934