[llvm] [VPlan][NFC] Add cost for `VPWidenMemoryRecipe`. (PR #105614)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

With this change, you should also be able to remove `VPWidenMemoryRecipe` from `getInstructionForCost` in `VPlan.cpp`

https://github.com/llvm/llvm-project/pull/105614