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

[David Blaikie via llvm-commits]

dwblaikie wrote:

(wonder if this generic code could use unit tests - this change could then be tested with an observer hash function that counts the number of calls, for instance)

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