[llvm] [DomTree] Make addChild/removeChild private, update NewGVN caller (PR #218164)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Note: if the contiguity requirement ever comes back, we can create a `SmallVector worklist`, append children to the vector and sort the tail by descending `RPOOrder[*]`, instead of mutating the dominator tree directly...

https://github.com/llvm/llvm-project/pull/218164