[llvm] [VPlan] Implement VPInterleaveRecipe::computeCost. (PR #106067)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> Where will this be used, do we already have tests that hit this code?

The VPlan-based cost model is used to pick the best vectorization factor already. For recipes that do not implement `::computeCost`, we currently fall back to the legacy cost model (via the instruction returned by `getInstructionForCost`.

With this patch, we use VPInterleaveRecipe::computeCost when computing the cost for all plans when selecting the best VF/Plan (and remove the need to retrieve the underlying instruction via `getInstructionForCost`)

https://github.com/llvm/llvm-project/pull/106067