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

[Elvis Wang via llvm-commits]

ElvisWang123 wrote:

> > With the change it should also be possible to remove `VPWidenMemoryRecipe` from `getInstructionForCost` in `VPlan.cpp`
> 
> Still missing?

Remove  `VPWidenMemoryRecipe` from `getInstructionForCost` will cause a test fail due to different VF between legacy model.
Maybe leave a TODO behind to revisit it when we don't need to compare to the legacy model?

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