[llvm] [NewGVN] Prevent cyclic dependencies by ensuring Leader has min RPO number (PR #82110)

[Alina Sbirlea via llvm-commits]

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@@ -393,7 +401,7 @@ class CongruenceClass {
   unsigned ID;
 
   // Representative leader.
-  Value *RepLeader = nullptr;
+  std::pair<Value *, unsigned int> RepLeader = {nullptr, ~0U};
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alinas wrote:

Same comment for RepLeader and for NextLeader below: add what the unsigned represents in a comment above each member.

https://github.com/llvm/llvm-project/pull/82110