[llvm] [MachineBasicBlock] Fix SlotIndexUpdater for insertion order (PR #69424)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Test case is in #69429 -- needs associated PHI Elimination changes.

So if you tried to apply #69429 without this patch, then we would see failures in some asan buildbots? If so, I think this patch is fine as-is.

> Unless you are suggesting I add a unit test?

I wasn't suggesting that.

https://github.com/llvm/llvm-project/pull/69424