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

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

I agree with nikic that this doesn't need an explicit test, "regressions" against this sort of implementation detail change should be monitored with compile time tracking, not functional tests, since that's ultimately what we care about in the context of this change. imo it's reasonable to assume that the existing domtree test coverage should cover everything here especially since this change didn't add any branches

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