[llvm] [MachineBasicBlock] Fix use after free in SplitCriticalEdge (PR #68786)

[via llvm-commits]

qcolombet wrote:

> I was wary that MachineFunction only admits a single delegate, so we might overlap with another user.
> However looking at it again, the only user I can see is GlobalISel, and this shouldn't overlap.
> I guess we could always extend MachineFunction in the future to allow multiple delegates.

Exactly!

Thanks for the fix!

https://github.com/llvm/llvm-project/pull/68786