[llvm] [DomTree] Provide a way to query if DFSInfo is valid (PR #91251)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I don't see why you'd need this; updateDFSNumbers short-circuits if the numbers are already up-to-date.

https://github.com/llvm/llvm-project/pull/91251