[VP] Add splitting for VP_STRIDED_STORE and VP_STRIDED_LOAD

Following the comment's thread of D117235, I added checks for the widening + splitting case, which also causes a split with one of the resulting vectors to be empty. Due to the same issues described in that same thread, the `fixed-vectors-strided-store.ll` test is missing the widening + splitting case, while the same case in the `strided-vpload.ll` test requires to manually split the loaded vector.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D121784