RSSA backend

[Matthew Fluet]

> RSSA is harder to understand than SSA.  I don't understand your point
> about using -keep-pass backend being wrong -- .backend.machine seems
> to exactly correspond exactly to the profile labels.

I thought that the cont/handler pair stubs were still being generated by
backend.fun directly in to Machine.Block.t's.  But, I see that they are
being done in the ssa-to-rssa conversion.  So, you're right that
Machine.Block.t's are in one-to-one correspondence with Rssa.Block.t's.

> Anyways, I am also happy with pushing back the profileInfo into RSSA.
> It can't be that hard.

That's my vote.  A quick look through the *.insert functions shows that it
shouldn't be too difficult to grab a useful profileInfo for each new
block.

> > Any reason not to mention the -color option in the profiling
> > documentation? 
> 
> Nope.  Go for it.  Also, reorder the -p entry in the man page so it is
> before -polyvariance.

Got it.