[llvm] [SelectionDAG] Stop storing EVTs in a function scoped static std::set. (PR #118715)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Is there some reason you're switching to FoldingSet, instead of just sticking with std::set?  Not sure what the performance of FoldingSet is like, but any performance difference is probably related to that switch, not whether the std::set is a global.

https://github.com/llvm/llvm-project/pull/118715