[LV] Use VScaleForTuning to fine-tune the cost per lane.

When targeting a specific CPU with scalable vectorization, the knowledge
of that particular CPU's vscale value can be used to tune the cost-model
and make the cost per lane less pessimistic.

If the target implements 'TTI.getVScaleForTuning()', the cost-per-lane
is calculated as:

  Cost / (VScaleForTuning * VF.KnownMinLanes)

Otherwise, it assumes a value of 1 meaning that the behavior
is unchanged and calculated as:

  Cost / VF.KnownMinLanes

Reviewed By: kmclaughlin, david-arm

Differential Revision: https://reviews.llvm.org/D113209