Implement CallArity analysis
This analysis finds out if a let-bound expression with lower manifest arity than type arity is always called with more arguments, as in that case eta-expansion is allowed and often viable. The analysis is very much tailored towards the code generated when foldl is implemented via foldr; without this analysis doing so would be a very bad idea! There are other ways to improve foldr/builder-fusion to cope with foldl, if any of these are implemented then this step can probably be moved to -O2 to save some compilation times. The current impact of adding this phase is just below +2% (measured running GHC's "make").
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