[VPlan] Compute cost for most opcodes in VPWidenRecipe (NFCI). (#98764)

Implement VPWidenRecipe::computeCost for most cases (except 
UDiv,SDiv,URem,SRem which require additional logic).

Note that this specializes `::computeCost` instead of `::cost`, as
`VPRecipeBase::cost` is responsible for skipping cost-computations
for pre-computed recipes for now.

The most recent version of the VPlan-based cost model introduction 
has been committed on Jul 10 (b841e2eca3b5c8b) and we should
probably give it at least a week in case additional mismatches surface.

PR: https://github.com/llvm/llvm-project/pull/98764

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