nll experiment: compute SCCs instead of iterative region solving
This is an attempt to speed up region solving by replacing the current iterative dataflow with a SCC computation. The idea is to detect cycles (SCCs) amongst region constraints and then compute just one value per cycle. The graph with all cycles removed is of course a DAG, so we can then solve constraints "bottom up" once the liveness values are known.
I kinda ran out of time this morning so the last commit is a bit sloppy but I wanted to get this posted, let travis run on it, and maybe do a perf run, before I clean it up.
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