[llvm] [LoopInterchange] Make the entries of the Dependency Matrix unique (PR #116195)

[Sjoerd Meijer via llvm-commits]

sjoerdmeijer wrote:

I have:
- removed the `fdef DUMP_DEP_MATRICIES`, replaced it with LLVM_DEBUG because it is indeed very useful to see this with debug enabled.
- replaced the unordered set with DenseSmallSet. The number of entries isn't very big normally, so I don't expect this to matter very much, but indeed DenseSmallSet is a bit better and also more consistent. 

In terms of overall compile-time, I don't expect this to make a big impact in the grand scheme of things. It may help a little, but might be difficult to measure. I see this more as a clean up. For one thing, just seeing the dependency matrix being dumped with -debug and not seeing all the duplicates is an improvement.

https://github.com/llvm/llvm-project/pull/116195