Study and document META II derivatives on a wiki page

[jmitchell]

META II is very elegant, but not ideal for everything. For example, META II's limit of 2 registers may make it difficult or impossible to emit single instructions which can have more than 2 parameters (consider ARM's condition code suffixes).

I'm specifically interested in whether META II has ever been extended to support more registers, but more generally interested in the trade offs of various meta compiler designs.