glsl: Make ir_variable::num_state_slots and ir_variable::state_slots private

glsl: Make ir_variable::num_state_slots and ir_variable::state_slots private

Also move num_state_slots inside ir_variable_data for better packing.

The payoff for this will come in a few more patches.

No change Valgrind massif results for a trimmed apitrace of dota2.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>