x86 backend update

[Stephen Weeks]

> Well, I'm still a bit away from spitting out code ready for the assembler,
> but I think I've got a good ammount of the framework down. Currently, I've
> written (most of) the translation from the MachineOutput IL to the
> pseudo-assembly IL. 

Looks great.  I have a couple of minor suggestions.

* Could you output the MachineOutput operands in the comments?

* It might be nice to write a separate pass that checks to see if a
  program your IL is valid X86 assembly.  Having this knowledge
  codified in one place would be good, and running this pass just
  before spitting out assembly would probably help to catch some errors.