nir: Make ballot intrinsics variable-size

nir: Make ballot intrinsics variable-size

This way they can return either a uvec4 or a uint64_t.  At the moment,
this is a no-op since we still always return a uint64_t.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>