together with the call arity analysis and the following patch (about inlining
maximum), we get nice benefits from fusing foldl and foldl' with good
producers:

                Min          -0.1%    -74.5%     -6.8%     -8.3%    -50.0%
                Max          +0.2%      0.0%    +38.5%    +38.5%      0.0%
     Geometric Mean          -0.0%     -4.1%     +7.7%     +7.7%     -0.8%

Because this depends on a compiler optimisation, we have to watch out for cases
where this is not an improvements, and whether they occur in the wild.