[llvm] [MachineBasicBlock] Fix use after free in SplitCriticalEdge (PR #66188)

[Carl Ritson via llvm-commits]

perlfu wrote:

Close this in favour of observer pattern in #68786.
(Created a fresh PR to avoid noise for all review groups added by files touched in this PR.)

https://github.com/llvm/llvm-project/pull/66188