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

[Florian Hahn via llvm-commits]

fhahn 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?

yes that would be great, together with an explanation that VPWidenMemoryRecipe case in getInstructionForCost still being needed for force-target-instruction handling.

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