nir: Use 'unsigned' instead of enum types in nir_variable::data

MSVC treats enums as signed, so storing values that use the topmost
bit of the explicitly sized field loads as a negative value instead.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6393>