Use Monoid in hptSomeThingsBelowUs

Sylvain Henryrequested to merge
hsyl20/ghc:hsyl20/hpt-monoid into master

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.

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