[LoopVectorizer] NFCI: Calculate register usage based on TLI.getTypeLegalizationCost.

This is more accurate than dividing the bitwidth based on the element count by the
maximum register size, as it can just reuse whatever has been calculated for
legalization of these types.

This change is also necessary when calculating register usage for scalable vectors, where
the legalization of these types cannot be done based on the widest register size, because
that does not take the 'vscale' component into account.

Reviewed By: SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D91059