[llvm] [DomTree] Reduce number of hash table lookups (NFC) (PR #73097)

[David Blaikie via llvm-commits]

dwblaikie wrote:

I get that it's somewhat an implementation detail (though observable through the public API - but not guaranteed) - but I think on the whole it could probably be a fairly stable test with useful signal, but maybe changes that affect the amount of hashing are more frequent/it's a less stable property than I'm estimating.

https://github.com/llvm/llvm-project/pull/73097