[LoopVectorize] NFC: Change selectVectorizationFactor to work on ElementCount.

This patch is NFC and changes occurrences of `unsigned Width`
and `unsigned i` to work on type ElementCount instead.

This patch is a preparatory patch with the ultimate goal of making
`computeMaxVF()` return both a max fixed VF and a max scalable VF,
so that `selectVectorizationFactor()` can pick the most cost-effective
vectorization factor.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D96019