Teach LV to use masked-store to support interleave-store-group with gaps (instead of scatters/scalarization).
The symmetric case of using masked-load to support interleaved-load-group with gaps was introduced a while ago, by https://reviews.llvm.org/D53668;
This patch completes the store-scenario leftover from D53668, and solves PR50566.
The above comment should be updated, along with its consequences...
In some cases, scaling might be able to handle gaps w/o masking, for both loads and stores; perhaps worth leaving behind a TODO.