[llvm] [mlir] [EquivalenceClasses] Use DenseMap instead of std::set. (NFC) (PR #134264)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> Are there any operator< we can drop now?

Couldnt find any more, most uses are either pointers or some variant of integers.

https://github.com/llvm/llvm-project/pull/134264