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

[Andrei Elovikov via llvm-commits]

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@@ -101,16 +101,11 @@ class DominanceFrontierBase {
 #endif
 
   void analyze(DomTreeT &DT) {
-    assert((IsPostDom || DT.root_size() == 1) &&
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eas wrote:

Why should we? We don't care about the number of roots here anymore, so if any of the users do, they should do the assert. Besides, `getRoot` at line 70 still has one.

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