[llvm] [ValueTracking] Handle recursive phis in knownFPClass (PR #114008)

[via llvm-commits]

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@@ -5999,6 +5999,21 @@ void computeKnownFPClass(const Value *V, const APInt &DemandedElts,
         if (IncValue == P)
           continue;
 
+        // If the Use is a select of this phi, use the fp class of the other
+        // operand to break the recursion.
+        Value *V;
+        if (match(IncValue, m_Select(m_Value(), m_Specific(P), m_Value(V))) ||
+            match(IncValue, m_Select(m_Value(), m_Value(V), m_Specific(P))))
+          IncValue = V;
+        // Same around 2-operand phi nodes
+        if (auto *IncPhi = dyn_cast<PHINode>(IncValue);
+            IncPhi && IncPhi->getNumIncomingValues() == 2) {
+          if (IncPhi->getIncomingValue(0) == P)
+            IncValue = IncPhi->getIncomingValue(1);
+          if (IncPhi->getIncomingValue(1) == P)
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goldsteinn wrote:

nit: `else if`

https://github.com/llvm/llvm-project/pull/114008