[llvm] [GVNSink] Fix non-determinisms by using Depth-First ordering (PR #90995)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic commented:

SplitBlockPredecessors invalidates the DFS numbering, I think.

Also, SplitBlockPredecessors changes the predecessor list of a basic block, which I think needs to invalidate any ModelledPHI referring to that block?

https://github.com/llvm/llvm-project/pull/90995