[llvm] [WIP][DAG] Add legalization handling for AVGCEIL/AVGFLOOR nodes (PR #92096)

[Jay Foad via llvm-commits]

================
@@ -5234,6 +5234,20 @@ SDValue DAGCombiner::visitAVG(SDNode *N) {
   if (N0 == N1 && Level >= AfterLegalizeTypes)
     return N0;
 
+  // Fold avgflooru(x,c) -> avgceilu(x,c-1)
+  // iff c > 0 and avgflooru isn't legal/custom but avgceilu is.
----------------
jayfoad wrote:

Is this a quirk of some specific target? Otherwise, why not do this for the signed cases too, and do the opposite fold from ceil -> floor?

https://github.com/llvm/llvm-project/pull/92096