Mainline space problem

[Joern Rennecke]

> If we try using a sparse bitmaps, then we are adding unnecessary 
> overhead for the most common case which will make the compiler run even 
> slower than it already does.

I'm not so sure about that.  Even for moderately sized problems, the
matrix tends to be sparse, and walking over all the pseudos that conflict
with one particular pseudo is an operation that takes a appreciable
part of the time accessing the data.
If that were not the case, mirroring the conflict matrix and using word
operations wouln't have yielded such a large improvement.

Thus, if we use a sparse matrix representation that has decent single-bit
access times and good row-walking performance, we might even speed up the
compiler for the common case.