[X86][CostModel] X86TTIImpl::getMemoryOpCost(): rewrite vector handling again

Instead of handling power-of-two sized vector chunks,
try handling the large vector in a stream mode,
decreasing the operational vector size
once it no longer works for the elements left to process.

Notably, this improves costs for overaligned loads - loading padding is fine.
This more directly tracks when we need to insert/extract the YMM/XMM subvector,
some costs fluctuate because of that.

Reviewed By: RKSimon, ABataev

Differential Revision: https://reviews.llvm.org/D100684