[IR] Split vscale_range interface

Interface is split from:

  std::pair<unsigned, unsigned> getVScaleRangeArgs()

into separate functions for min/max:

  unsigned getVScaleRangeMin();
  Optional<unsigned> getVScaleRangeMax();

Reviewed By: sdesmalen, paulwalker-arm

Differential Revision: https://reviews.llvm.org/D114075