libcgc: Disassemble dot3 and do4 opcodes

3 and 4 component dot-products seems to be separate opcodes, so
let's disassemble them. These revealed that the opcode-field was
a bit wider that previously suspected.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
libcgc: try to decode rcp and rsq

This seems a bit wonky, but it might be correct; so far these are
the only opcodes that only takes a scalar as an argument. So
perhaps they are implemented differently than the rest.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>