Use Monoid in hptSomeThingsBelowUs
Found while doing an unrelated refactoring. It seems to have a moderate but good impact on perf tests in CI (https://gitlab.haskell.org/hsyl20/ghc/-/pipelines/44454). In particular:
MultiLayerModules(normal) ghc/alloc 3125771138.7 3065532240.0 -1.9%
So it's likely that huge projects will benefit from this.