This patch makes sure that the loopvectorizer generates VPlans for
all power of two VFs upto the provided maximum VFs (can be both fixed
and/or scalable). This patch is intended to be a non-functional change,
because selectVectorizationFactor does not automatically pick any of
the scalable VPlans yet.
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