nir: Fix overlapping vars in nir_assign_io_var_locations()

When handling two variables with overlapping locations, we process the one with lower location first, and then extend the location -> driver_location map to guarantee that it's contiguous for the second variable too. But the loop had the wrong bound, so we weren't extending the map 100%, which could lead to problems later such as an incorrect num_inputs. The loop index i is an index into the slots of the variable, so we need to stop at the final slot of the variable (var_size) instead of the number of unassigned slots.

This fixes spec@arb_enhanced_layouts@execution@component-layout@vs-fs-array-interleave-range on radeonsi NIR.