[llvm] [LLVM][SelectionDAG] Add poison/undef folds for signed/unsigned max/min. (PR #149334)

[Matt Arsenault via llvm-commits]

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@@ -7875,13 +7875,22 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
 
   // Fold a bunch of operators when the RHS is undef.
   if (N2.isUndef()) {
-    switch (Opcode) {
-    case ISD::XOR:
-      if (N1.isUndef())
+    if (N1.isUndef()) {
+      switch (Opcode) {
+      case ISD::XOR:
         // Handle undef ^ undef -> 0 special case. This is a common
         // idiom (misuse).
         return getConstant(0, DL, VT);
-      [[fallthrough]];
+      case ISD::SMAX:
+      case ISD::SMIN:
+      case ISD::UMAX:
+      case ISD::UMIN:
+        return N2.getOpcode() == ISD::POISON ? getPOISON(VT) : getUNDEF(VT);
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arsenm wrote:

This is equivalent to just returning N2? 

https://github.com/llvm/llvm-project/pull/149334