[llvm] [SelectionDAG] Introduce ISD::PTRADD (PR #140017)

[Matt Arsenault via llvm-commits]

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@@ -2617,6 +2620,85 @@ SDValue DAGCombiner::foldSubToAvg(SDNode *N, const SDLoc &DL) {
   return SDValue();
 }
 
+/// Try to fold a pointer arithmetic node.
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arsenm wrote:

Maybe should split this into a PR with tests and a target use 

https://github.com/llvm/llvm-project/pull/140017