pco, pygen: rework isa gen to support multi-instruction groups

- Split encode and group mappings to allow the former to be re-used.
- Add custom zero value mapping for bitset enums.
- Enable optional enum mapping for ref mods (previously just op mods).
- Commonize nop/nop.end.

Signed-off-by: Simon Perretta <simon.perretta@imgtec.com>
Acked-by: Frank Binns <frank.binns@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33998>
10 files changed