mesa/st/glsl_to_tgsi: Add helper class for array live range merging and interleaving

mesa/st/glsl_to_tgsi: Add helper class for array live range merging and interleaving

This class holds the array length, live range, and accessed components, and
it implements the logic for evaluating how arrays are merged and interleaved.

v4: - Add logic to evaluate merge and interleave of a pair of arrays to
      the class array_live_range.
    - document class
    - update commit message

Thanks Nicolai Hähnle for the pointers given.

Signed-off-by: Gert Wollny <gw.fossdev@gmail.com>
Acked-by: Dave Airlie <airlied@redhat.com>