[LV] Calculate max feasible scalable VF.

This patch also refactors the way the feasible max VF is calculated,
although this is NFC for fixed-width vectors.

After this change scalable VF hints are no longer truncated/clamped
to a shorter scalable VF, nor does it drop the 'scalable flag' from
the suggested VF to vectorize with a similar VF that is fixed.

Instead, the hint is ignored which means the vectorizer is free
to find a more suitable VF, using the CostModel to determine the
best possible VF.

Reviewed By: c-rhodes, fhahn

Differential Revision: https://reviews.llvm.org/D98509