There is a special 64-bit clause type which can only consist of
instructions acting on 64-bit registers. The register field of each
instruction in a 64-bit clause is interpreted entirely differently.
We extend RegCtrl to include everything in the original Regs bitfield,
so that we don't have to poke the original bitfield after decoding it,
since that wouldn't work for 64-bit clauses. We then add a new Regs64
bitfield and DecodeRegCtrl64() function to decode the new format, and
make sure to call it for 64-bit clauses.