[llvm] Matched some basic ISD::AVGFLOORU patterns (PR #84903)

[Simon Pilgrim via llvm-commits]

================
@@ -2821,6 +2821,38 @@ SDValue DAGCombiner::visitADDLike(SDNode *N) {
   return SDValue();
 }
 
+// Attempt to form ext(avgflooru(A, B)) from add(and(A, B), lshr(xor(A, B), 1))
+static SDValue combineFixedwidthToAVG(SDNode *N, SelectionDAG &DAG) {
+  assert(N->getOpcode() == ISD::ADD && "ADD node is required here");
+  SDValue And = N->getOperand(0);
+  SDValue Lshr = N->getOperand(1);
+  if (And.getOpcode() != ISD::AND || Lshr.getOpcode() != ISD::SRL)
----------------
RKSimon wrote:

This is the kind of thing I had in mind:
```cpp
  if (TLI.isOperationLegal(ISD::AVGFLOORU, VT)) {
    SDValue A, B;
    if (sd_match(N, m_Add(m_And(m_Value(A), m_Value(B)),
                          m_Srl(m_Xor(m_Deferred(A), m_Deferred(B)),
                                m_SpecificInt(1))))) {
      return DAG.getNode(ISD::AVGFLOORU, DL, VT, A, B);
    }
  }
```

https://github.com/llvm/llvm-project/pull/84903