Instruction selector internals

It's a pattern matcher operating on DAG's. Target independent nodes
in, target specific nodes out.

I guess this is general enough. So the restriction lies in the implementation
language and not the pattern matcher itself. I'll spend some (a lot of) time to
understand the backend-specific issues that are of particular interest to me.
Eventually, I'll get back with a patch (hopefully :slight_smile: and before that with more
questions :slight_smile:

Nikolaos Kavvadias