[llvm] [DomTree] Prepend children instead of appending (PR #218178)

[Fangrui Song via llvm-commits]

MaskRay wrote:

[No l-c-t-t significance as expectd](https://llvm-compile-time-tracker.com/compare.php?from=6e2ed23e118954dbf0f7cc2e9de672fdccc130c9&to=ae8c08916450799ff22049875442aa793d4b9c75&stat=instructions%3Au)

https://github.com/llvm/llvm-project/pull/218178