RSSA backend

[Stephen Weeks]

> > One other strangeness, I ran all the benchmarks comparing the current
> > stable MLton and the (just checked in) RSSA version, both using the C
> > codegen.  Weirdly enough, the RSSA version was noticeable faster in
> > most cases.  I have no explanation.  Here are the timings.
> 
> You mentioned earlier that there is a new pseudo-register allocator.  That
> might be enough for gcc to a little more (or less) with the code.

Maybe, but the new allocator is more affecting (MLton) stack slot
assignment, which gc probably can't do much with.  Let's wait and see
how the x86-codegens compare and see if there is a difference for
them.