[llvm] [BasicBlockUtils] Fix SplitBlockPredecessors incorrect dominator insert (PR #107190)

[Joshua Cao via llvm-commits]

================
@@ -1160,7 +1160,10 @@ static void UpdateAnalysisInformation(BasicBlock *OldBB, BasicBlock *NewBB,
       // Split block expects NewBB to have a non-empty set of predecessors.
       SmallVector<DominatorTree::UpdateType, 8> Updates;
       SmallPtrSet<BasicBlock *, 8> UniquePreds;
-      Updates.push_back({DominatorTree::Insert, NewBB, OldBB});
+      if (OldBB->getSinglePredecessor()) {
+        assert(OldBB->getSinglePredecessor() == NewBB);
+        Updates.push_back({DominatorTree::Insert, NewBB, OldBB});
+      }
----------------
caojoshua wrote:

Yes, we do always update a CFG edge. But that might not be a **dominating** edge. In the example above (`bb2` is `OldBB`):

```
bb0:
  br label %new_bb
new_bb:
  br label %bb2
bb1:
  br label %bb2
bb2: ...
```

Without this patch, this would have added a edge from `new_bb` to `bb2`. But `new_bb` does not actually dominate `bb2`, since we can reach `bb2` from another path `bb1`

https://github.com/llvm/llvm-project/pull/107190