This patch adds a 'enable-scalable-vectorizaton' flag to the
loopvectorizer, which for now doesn't change the vectorization, but it
let's computeMaxVF call computeFeasibleMaxVF with 'ComputeScalableMaxVF =
true', which prints the output to a DEBUG ostream. This is then verified
by the corresponding test.
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