NFC: cleaned up and renamed scalable-vf-analysis.ll -> scalable-vectorization.ll

* Removes unnecessary loop hints.
* Use RUN line with '-scalable-vectorization=preferred' instead of 'on'
  for the maximize-bandwidth behaviour. This prepares the test for enabling
  scalable vectorization; With a forced instruction-cost of 1, 'on' will
  always favour fixed-width VF to be chosen, whereas with 'preferred'
  we can check that the maximize-bandwidth option in combination with
  scalable-vectorization=preferred actually picks a scalable VF.
* Renamed to scalable-vectorization.ll, because a follow-up patch will
  test more than just analysis.