return the rank (starting at 0) at which the
specified color was produced
this is formally equivalent to getUsedColors().IndexOf(color)
but may be quicker in some cases
experimental section: for gc
this method may be called when the optimizer will never be used again
it will allow to release some internal field for the garbage collector.
A graph compressor is used to compress vertices and edges of graphs.
It is based on a compression condition that is expressed by
a subsumption relation S between vertices.
v S u => u can be removed from graph, u is stored in Alias(v), v is stored as Root(u).