Improve performance of some Tree folds

[meooow25]

This performance issue is avoided in Tree's foldr already. This change applies the improvement in a few other places, and adds an explanatory comment.

Ran into this with #1244.

[meooow25]

Benchmarks with GHC 9.14:

Name                                                           Time - - - - - - - -    Allocated - - - - -
                                                                    A       B     %         A       B     %
PostOrder.Foldable.folds.bin,n=1000.foldr_cpsOneShotSum        8.1 μs  4.5 μs  -44%    117 KB   78 KB  -33%
PostOrder.Foldable.folds.bin,n=1000.foldr_cpsSum               8.2 μs  7.8 μs   -4%    117 KB  101 KB  -13%
PostOrder.Foldable.folds.bin,n=1000.foldr_elem                 4.8 μs  2.7 μs  -44%     78 KB   39 KB  -50%
PostOrder.Foldable.folds.bin,n=1000.foldr_traverseSum          6.9 μs  6.2 μs   -9%     86 KB   70 KB  -18%
PostOrder.Foldable.folds.bin,n=1000000.foldr_cpsOneShotSum      24 ms  5.9 ms  -75%    114 MB   76 MB  -33%
PostOrder.Foldable.folds.bin,n=1000000.foldr_cpsSum             24 ms   23 ms   -5%    114 MB   99 MB  -13%
PostOrder.Foldable.folds.bin,n=1000000.foldr_elem              7.9 ms  3.7 ms  -53%     76 MB   38 MB  -49%
PostOrder.Foldable.folds.bin,n=1000000.foldr_traverseSum        21 ms   21 ms   -2%     84 MB   69 MB  -18%
PostOrder.Foldable.folds.line,n=1000.foldr_cpsOneShotSum       8.2 μs  5.3 μs  -35%    117 KB   78 KB  -33%
PostOrder.Foldable.folds.line,n=1000.foldr_cpsSum              8.3 μs  8.4 μs   +0%    117 KB  101 KB  -13%
PostOrder.Foldable.folds.line,n=1000.foldr_elem                6.2 μs  3.4 μs  -44%     78 KB   39 KB  -50%
PostOrder.Foldable.folds.line,n=1000.foldr_traverseSum         6.8 μs  6.6 μs   -3%     86 KB   70 KB  -18%
PostOrder.Foldable.folds.line,n=1000000.foldr_cpsOneShotSum     53 ms   38 ms  -28%    114 MB   76 MB  -33%
PostOrder.Foldable.folds.line,n=1000000.foldr_cpsSum            52 ms   39 ms  -25%    114 MB   99 MB  -13%
PostOrder.Foldable.folds.line,n=1000000.foldr_elem              81 ms   22 ms  -72%     76 MB   38 MB  -49%
PostOrder.Foldable.folds.line,n=1000000.foldr_traverseSum       51 ms   38 ms  -26%     84 MB   69 MB  -18%
PostOrder.Foldable1.toNonEmpty.bin,n=1000                      4.5 μs  3.8 μs  -14%     70 KB   55 KB  -22%
PostOrder.Foldable1.toNonEmpty.bin,n=1000000                    19 ms   17 ms   -9%     69 MB   53 MB  -22%
PostOrder.Foldable1.toNonEmpty.line,n=1000                     4.3 μs  4.5 μs   +4%     62 KB   62 KB   +0%
PostOrder.Foldable1.toNonEmpty.line,n=1000000                   52 ms   52 ms   +1%     61 MB   61 MB   +0%
edges.bin,n=1000                                               6.9 μs  5.4 μs  -21%    148 KB  140 KB   -5%
edges.bin,n=1000000                                             34 ms  6.5 ms  -80%    145 MB  137 MB   -5%
edges.line,n=1000                                              7.8 μs  6.3 μs  -19%    148 KB  140 KB   -5%
edges.line,n=1000000                                            44 ms   25 ms  -42%    145 MB  137 MB   -5%
leaves.bin,n=1000                                              4.1 μs  3.5 μs  -14%     59 KB   43 KB  -26%
leaves.bin,n=1000000                                            11 ms  4.5 ms  -58%     57 MB   42 MB  -26%
leaves.line,n=1000                                             5.1 μs  3.3 μs  -35%     62 KB   31 KB  -49%
leaves.line,n=1000000                                           36 ms   32 ms  -12%     61 MB   31 MB  -50%