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

[Carl Ritson via llvm-commits]

perlfu wrote:

> Could we fix `SplitCriticalEdges` instead of pushing the burden of the SlotIndex update to all backends?

I am open to alternatives.
>From my perspective it is this, or add some kind of listener pattern to `MachineBasicBlock` so `SplitCriticalEdges` can intercept instructions added or removed by target specific code.

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