libcgc: rework fragment-shader ALU operands

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
libcgc: decode accumulation-flag

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
libcgc: decode SFU instructions

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