(comp): GameBoyInstructionSet

[arkamnite]

Complete GameBoy Instruction Set description.

It will naturally take a very long time to complete the entire instruction set description, and there are likely to be clashes with the existing AVR instructions and lowering those to DAG. There are specific key tasks which must be at least formalised, before the existing implementations are modified:

1. Devise a calling convention; refer to SDCC convention if needed.
2. Categorise instructions into instruction formats; at least determine an exhaustive list of these formats.
3. Investigate pseudo-instructions.

The necessary files to be modified include:

• GameBoyInstrFormats.td
• GameBoyInstrInfo.td
• GameBoyInstrInfo.cpp