RSSA

[Matthew Fluet]

> 5. Translate to MACHINE, which mostly involves replacing Rssa operands
>    with Machine operands and inserting liveness information.
> 6. Translate to MACHINE_OUTPUT, which does some cleanup, and maybe
>    should be combined with the previous pass.

This distinction arose when we first added the x86-codegen.  Essentially,
it lifted a bunch of stuff from the c-codegen that needed to be computed
for both codegens.  I've got no objections to trying to combine the two;
some of the clean up is just eliminating accumulation refs, which probably
doesn't need to happen.