[llvm] [X86] Resolve FIXME: Accept live flag if no overflow occurs (PR #86836)

[Simon Pilgrim via llvm-commits]

================
@@ -54539,9 +54552,14 @@ static SDValue combineADC(SDNode *N, SelectionDAG &DAG,
   }
 
   // Fold ADC(ADD(X,Y),0,Carry) -> ADC(X,Y,Carry)
-  // iff the flag result is dead.
+  // iff the flag result is dead or if X + Y are known to not overflow.
   if (LHS.getOpcode() == ISD::ADD && RHSC && RHSC->isZero() &&
-      !N->hasAnyUseOfValue(1))
+      (!N->hasAnyUseOfValue(1) ||
+       (DAG.computeOverflowForSignedAdd(LHS.getOperand(0), LHS.getOperand(1)) ==
+            SelectionDAG::OverflowKind::OFK_Never &&
+        DAG.computeOverflowForUnsignedAdd(LHS.getOperand(0),
+                                          LHS.getOperand(1)) ==
+            SelectionDAG::OverflowKind::OFK_Never)))
----------------
RKSimon wrote:

```
willNotOverflowAdd(true, LHS.getOperand(0), LHS.getOperand(1)) &&
willNotOverflowAdd(false, LHS.getOperand(0), LHS.getOperand(1))
```

https://github.com/llvm/llvm-project/pull/86836