[NFCI] [LoopIdiom] Let processLoopStridedStore take StoreSize as SCEV instead of unsigned

Letting it take SCEV allows further modification on the function to optimize
if the StoreSize / Stride is runtime determined.

This is a preceeding of D107353.
The big picture is to let LoopIdiom deal with runtime-determined sizes.

Reviewed By: Whitney, lebedev.ri

Differential Revision: https://reviews.llvm.org/D104595