If you're interested in the testing techniques used by established open source software, here are a couple projects that appear to have far more extensive test coverage than LLVM:
I don't mean it as criticism of the LLVM project, as I haven't worked with it and can't say whether its tests are sufficient. Only that, from reading this overview, its testing techniques do not seem particularly elaborate.
SQLite ( https://sqlite.org/testing.html )
Opus ( https://www.ietf.org/proceedings/82/slides/codec-4.pdf )
I don't mean it as criticism of the LLVM project, as I haven't worked with it and can't say whether its tests are sufficient. Only that, from reading this overview, its testing techniques do not seem particularly elaborate.