[llvm] [Utils] Fix incorrect LCSSA PHI nodes when splitting critical edges with MergeIdenticalEdges (PR #131744)

[Nikita Popov via llvm-commits]

nikic wrote:

It looks like the test is failing:
```

C:\ws\src\llvm\unittests\Transforms\Utils\BasicBlockUtilsTest.cpp:492
Expected equality of these values:
  nullptr
    Which is: NULL
  PN
    Which is: 0000020DEA46FCA8
```

https://github.com/llvm/llvm-project/pull/131744