[llvm] [DomTree] Remove support for unnumbered graphs (PR #212026)

[Alexis Engelke via llvm-commits]

aengelke wrote:

[stage2-O3 -0.07%](https://llvm-compile-time-tracker.com/compare.php?from=04dd04da2e664eea61ab2e17775148ffc21e29ae&to=b73d1d4fff4ff18d0e47129bcd0638eed3d07f03&stat=instructions:u) due to avoiding the +1 for pre-dominator trees.

https://github.com/llvm/llvm-project/pull/212026