A wild use case of union over a variant because we don't want to
increase the encoding size from 128-bit to 256-bit (because of padding).
The type of operation is encoded with the table operation type, so a
variant is unnecessary and we get to keep the 128-bit encoding.
This allows us to have "recursive" x86 table descriptions. But in
reality this is going to only be one layer deep. As this will allow the
Frontend decoder to select instruction encodings based on arch bitness
once the tables are generated correctly.