glsl: Collect all of the non-constant index error checks together

glsl: Collect all of the non-constant index error checks together

This puts all of the checks togeher for easier reading.  It also means
that all the checks are blocked on array->type->is_array.  Shortly this
will allow elimination of some is_error check work-arounds in this
function.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>