[llvm] [DominanceFrontier] Support multiple root nodes for post-dom (PR #181257)

[Luke Lau via llvm-commits]

================
@@ -90,13 +95,16 @@ void DominanceFrontierBase<BlockT, IsPostDom>::calculate(
     BlockT *parentBB = currentW->parentBB;
     const DomTreeNodeT *currentNode = currentW->Node;
     const DomTreeNodeT *parentNode = currentW->parentNode;
-    assert(currentBB && "Invalid work object. Missing current Basic Block");
     assert(currentNode && "Invalid work object. Missing current Node");
-    DomSetType &S = this->Frontiers[currentBB];
+
+    // Note that for `IsPostDom == true`, virtual root node (empty currentBB)
----------------
lukel97 wrote:

Nit

```suggestion
    // Note that for `IsPostDom == true`, the virtual root node (null currentBB)
```

https://github.com/llvm/llvm-project/pull/181257