pan/bi: Add XML describing the instruction set

pan/bi: Add XML describing the instruction set

Throughout this series, this XML file will serve as architectural ground
truth. It contains every instruction in the instruction set with all
programmable modifiers, as well as logic for computing derived values
(indirectly specified modifiers) and swapping operands as needed by
numerous encodings. It also allows for multiple encodings per
instruction differentiated by exact bits (a generalization of opcodes),
with different derived fields in each encoding, and logic tests to
select between the encodings at pack time.

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>