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

[Florian Hahn via llvm-commits]

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@@ -340,6 +340,9 @@ class LoopVectorizationPlanner {
   /// A builder used to construct the current plan.
   VPBuilder Builder;
 
+  /// Computes the cost of \p Plan for vectorization factor \p VF.
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fhahn wrote:

The current implementation requires access to the legacy cost model and I kept the various computeCost implementations that still require it separate from the VPlan-only infrastructure, to avoid leaking/polluting the VPlan-only cost implementations. I'd propose moving it to `VPlan::computeCost` once the legacy cost model isn't needed any longer.

Added a comment + TODO

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