[new-regalloc] Dataflow too slow...

[Michael Hayes]

Geert Bosch writes:
 > One of the things I'm stumbling on now is that df_rd_global_compute
 > takes way too much time to allow me to bootstrap the Ada compiler.

I used a simple worklist algorithm for this to get the ball rolling.
The usual solution to impove the performance is to order the blocks
with a depth first search.  This was on my todo list.

There are many other more esoteric algorithms in the literature which
would be a nice self-contained project for someone to implement.

Michael.