MLIR is a Google/Tensorflow project, not LLVM, and AFAICT is only used by TensorFlow. This might be because it was only released a few months ago, but also from skimming through their "write a language" tutorial it seems it provides barely anything useful, just JSON-like read/write of the IR, an LLVM output pass which is marginally simpler than writing it yourself, and abstraction machinery to do generic SSA operations and share "dialects" with other DSLs.

I wouldn't say it's worthless, since Tensorflow is using it and MLIR could grow into the middle-end for some new HPC / data science language, but personally I am somewhat leery. GHC already has Core, STG, and C-- so it doesn't particularly need a new intermediate language, and for a new language I would prefer to investigate something besides SSA like Thorin (https://anydsl.github.io/Thorin.html).