glsl: use bitwise operators in varying_matches::compute_packing_class()

glsl: use bitwise operators in varying_matches::compute_packing_class()

The mix of bitwise operators with * and + to compute the packing_class
values was a little weird.  Just use bitwise ops instead.

v2: add assertion to make sure interpolation bits fit without collision,
per Timothy.  Basically, rewrite function to be simpler.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>