Improve GraphColor.colorScan

Improve GraphColor.colorScan

Testing whether a node in the conflict graph is trivially
colorable (triv) is still a somewhat expensive operation.

When we find a triv node during scanning, even though we remove
it and its edges from the graph, this is unlikely to to make the
nodes we've just scanned become triv - so there's not much point
re-scanning them right away.

Scanning now takes place in passes. We scan the whole graph for
triv nodes and remove all the ones found in a batch before rescanning
old nodes.

Register allocation for SHA1.lhs now takes (just) 40% of total
compile time with -O2 -fregs-graph on x86