[SLP] Honor min/max regsize and min/max VF in vectorizeStores

Make sure we use PowerOf2Floor instead of PowerOf2Ceil when
calculating max number of elements that fits inside a vector
register (otherwise we could end up creating vectors larger
than the maximum vector register size).

Also make sure we honor the min/max VF (as given by TTI or
cmd line parameters) when doing vectorizeStores.

Reviewed By: anton-afanasyev

Differential Revision: https://reviews.llvm.org/D97691