[DAGCombiner][RISCV] Canonicalize (bswap(bitreverse(x))->bitreverse(bswap(x)).

If the bitreverse gets expanded, it will introduce a new bswap. By
putting a bswap before the bitreverse, we can ensure it gets cancelled
out when this happens.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D118012

Author: topperc

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -9375,6 +9375,17 @@ SDValue DAGCombiner::visitBSWAP(SDNode *N) {
   // fold (bswap (bswap x)) -> x
   if (N0.getOpcode() == ISD::BSWAP)
     return N0->getOperand(0);
+
+  // Canonicalize bswap(bitreverse(x)) -> bitreverse(bswap(x)). If bitreverse
+  // isn't supported, it will be expanded to bswap followed by a manual reversal
+  // of bits in each byte. By placing bswaps before bitreverse, we can remove
+  // the two bswaps if the bitreverse gets expanded.
+  if (N0.getOpcode() == ISD::BITREVERSE && N0.hasOneUse()) {
+    SDLoc DL(N);
+    SDValue BSwap = DAG.getNode(ISD::BSWAP, DL, VT, N0.getOperand(0));
+    return DAG.getNode(ISD::BITREVERSE, DL, VT, BSwap);
+  }
+
   return SDValue();
 }