[NFC][LoopIdiom] Let processLoopStoreOfLoopLoad 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.

The plan is to let memcpy / memmove deal with runtime-determined sizes, just
like what D107353 did to memset.

Reviewed By: bmahjour

Differential Revision: https://reviews.llvm.org/D108289