Fix an long-standing bug in OccurAnal
This bug was beautifully characterised in Trac #12776, which showed a small program for which the inliner went into an infinite loop. Eeek. It turned out to be a genuine and long-standing bug in the occurrence analyer, specifically in the bit that identifies loop breakers. In this line pairs | isEmptyVarSet weak_fvs = reOrderNodes 0 bndr_set weak_fvs tagged_nodes  | otherwise = loopBreakNodes 0 bndr_set weak_fvs loop_breaker_edges  the 'tagged_nodes' should be 'loop_breaker_edges'. That's it! The diff looks a lot bigger because I did some work on comments and variable naming, but that's all it is. We were using the wrong set of dependencies! I'm astonished that this bug has not caused more trouble. It dates back to at least 2011 and maybe further.
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