[llvm] [SelectionDAG] Resolve TODO: Use signedAddMayOverflow (PR #85586)

[Matt Arsenault via llvm-commits]

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@@ -4125,8 +4125,13 @@ SelectionDAG::computeOverflowForSignedAdd(SDValue N0, SDValue N1) const {
   if (ComputeNumSignBits(N0) > 1 && ComputeNumSignBits(N1) > 1)
     return OFK_Never;
 
-  // TODO: Add ConstantRange::signedAddMayOverflow handling.
-  return OFK_Sometime;
+  KnownBits N0Known = computeKnownBits(N0);
+  KnownBits N1Known = computeKnownBits(N1);
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arsenm wrote:

can early exit if first query didn't report anything useful 

https://github.com/llvm/llvm-project/pull/85586