[LV] Adjust cost model to use uniform store lowering for unpredicated uniform stores

Follow up to D133580; adjust the cost model to prefer uniform store lowering for scalable stores which are unpredicated.

The impact here isn't in the uniform store lowering quality itself. InstCombine happily converts the scatter form into the single store form. The main impact is in letting the rest of the cost model make choices based on the knowledge that the vector will be scalarized on use.

Differential Revision: https://reviews.llvm.org/D134460