[SelectionDAG][VP] Add splitting/widening for VP_LOAD and VP_STORE

Original patch by @hussainjk.

This patch was split off from D109377 to keep vector legalization
(widening/splitting) separate from vector element legalization
(promoting).

While the original patch added a third overload of
SelectionDAG::getVPStore, this patch takes the liberty of collapsing
those all down to 1, as three overloads seems excessive for a
little-used node.

The original patch also used ModifyToType in places, but that method
still crashes on scalable vector types. Seeing as the other VP
legalization methods only work when all operands need identical
widening, this patch follows in that vein.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D117235