[VPlan] Support scalable vectors in outer-loop vectorization

This patch enables scalable vectors in the VPlan-native path.
If a vectorization factor is specified via loop vectorization hints,
that factor is used. If no vectorization factor is specified, but the
target preferes scalable vectorization, a scalable vectorization factor
is selected.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D157484