isaspec: Add initial decoding support

isaspec: Add initial decoding support

This reuses the <map> entries in the <encode> block to go in the reverse
direction and parse an instruction into a machine-readable structure. It
currently assumes that <map> entries are simple l-values like
"src->src[0]" or "src->flag", which is enough for afuc, but the plan for
the future is to use the <decode> block to allow us to override that for
more complex cases.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23949>