[llvm] [SelectionDAG] Preserve poison for abs INT_MIN lowering (PR #183851)

[Simon Pilgrim via llvm-commits]

================
@@ -12109,6 +12112,54 @@ SDValue DAGCombiner::visitABS(SDNode *N) {
   return SDValue();
 }
 
+SDValue DAGCombiner::visitABS_MIN_POISON(SDNode *N) {
+  SDValue N0 = N->getOperand(0);
+  EVT VT = N->getValueType(0);
+  SDLoc DL(N);
+
+  // fold (abs_min_poison c1) -> c2 (or poison if c1 == INT_MIN)
+  if (SDValue C =
+          DAG.FoldConstantArithmetic(ISD::ABS_MIN_POISON, DL, VT, {N0}))
+    return C;
+  // fold (abs_min_poison (abs_min_poison x)) -> (abs_min_poison x)
+  // fold (abs_min_poison (abs x)) -> (abs x)
+  if (ISD::isAbsOpcode(N0.getOpcode()))
+    return N0;
+  // fold (abs_min_poison (freeze (abs x))) -> (freeze (abs x))
+  // fold (abs_min_poison (freeze (abs_min_poison x))) -> (freeze (abs_min_poison x))
+  // Valid because: for x != INT_MIN both sides equal abs(x); for x == INT_MIN
+  // both forms produce a non-deterministic but well-defined value since freeze
+  // already consumed the poison.
+  if (N0.getOpcode() == ISD::FREEZE &&
+      ISD::isAbsOpcode(N0.getOperand(0).getOpcode()))
+    return N0;
----------------
RKSimon wrote:

You could merge this with the previous fold with:
```suggestion
  if (ISD::isAbsOpcode(peekThroughFreeze(N0).getOpcode()))
    return N0;
```

https://github.com/llvm/llvm-project/pull/183851