New branch opened: dataflow-branch

[Daniel Berlin]

This branch contains the work Ken Zadeck and I have been doing  
replacing the backend dataflow (such as live register analysis) with  
df.c based dataflow.

It currently bootstraps on x86-linux-gnu, and powerpc-linux-gnu, and  
powerpc-darwin.

Currently, reg-stack doesn't like accurate liveness info (IE with  
unset variables removed), so that the more accurate liveness analysis  
is disabled at some points in the compiler.

The branch may be slower or faster than mainline right now, we  
haven't concetrated on timings, but we are confident it will be  
faster when we are done.

The branch produces about 1% better spec scores, due to better  
liveness information being computed.

I will document the branch in wwwdocs soon.