[llvm] [NFC][DominanceFrontier] Replace std::map with DenseMap for DomSetMapType (PR #70403)

[Wenju He via llvm-commits]

wenju-he wrote:

@kuhar could you please help to review? Thank you very much. I don't have permission to add you as reviewer.

https://github.com/llvm/llvm-project/pull/70403