pan/bi: Add packing generator

pan/bi: Add packing generator

From the ISA definition, we can generate a function for each instruction
that looks at the bi_instruction in the intermediate representation and
emits a 20- or 23-bit word (for ADD/FMA respectively) containing that
instruction with all of its modifiers.

These will approximate the old packing routines, although the mapping of
bi_instruction to machine instructions will be hardcoded (at least for
now).

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6749>