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

[Elvis Wang via llvm-commits]

ElvisWang123 wrote:

When removing `VPWidenMemoryRecipe` from the `getInstructionForCost()` will cause test fails.
Because in the `VPRecipeBase::cost()` , there is a check for override the instruction cost by option needs the return from `getInstructionForCost()` .
```
if (UI && ForceTargetInstructionCost.getNumOccurrences() > 0 &&
      RecipeCost.isValid())
    RecipeCost = InstructionCost(ForceTargetInstructionCost);
```

Should we just remove the check of `UI` ?

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