[llvm] [SDAG] Handle insert_subvector in isKnownNeverNaN (PR #131989)

[Matt Arsenault via llvm-commits]

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@@ -5625,6 +5625,9 @@ bool SelectionDAG::isKnownNeverNaN(SDValue Op, bool SNaN, unsigned Depth) const
            (SNaN && !C->getValueAPF().isSignaling());
   }
 
+  if (Op.isUndef())
+    return true;
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arsenm wrote:

This is incorrect. You could do this for poison, but not undef tttt

https://github.com/llvm/llvm-project/pull/131989