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

[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.
----------------
goldsteinn wrote:

Unless there is certainty that no other target can benefit, imo its written generically enough to keep here.

I.e the general pattern: `if(NotLegal(A) && IsLegal(B) && MayTransform(A, B)) { return B; }` is sensible as a generic combine, even if the motivation was one particular target.

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