These tests deliberately use non-standard formatting, so that the line
execution counts reported by `llvm-cov` reveal additional information about
where code regions begin and end.
In some cases we need to prepare a coverage expression that is the sum of an
arbitrary number of other terms. This patch simplifies the code paths that
build those sums.
This causes some churn in the mappings, because the previous code was building
its sums in a somewhat idiosyncratic order.