[TargetLowering][RISCV] Prevent scalarization of fixed vector bswap.

It's better to do the ands, shifts, ors in the vector domain than
to scalarize it and do those operations on each element.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D112248

llvm/lib/CodeGen/SelectionDAG/LegalizeVectorOps.cpp

@@ -1101,13 +1101,23 @@ SDValue VectorLegalizer::ExpandBSWAP(SDNode *Node) {
   EVT ByteVT = EVT::getVectorVT(*DAG.getContext(), MVT::i8, ShuffleMask.size());
 
   // Only emit a shuffle if the mask is legal.
-  if (!TLI.isShuffleMaskLegal(ShuffleMask, ByteVT))
-    return DAG.UnrollVectorOp(Node);
-
+  if (TLI.isShuffleMaskLegal(ShuffleMask, ByteVT)) {
     SDLoc DL(Node);
     SDValue Op = DAG.getNode(ISD::BITCAST, DL, ByteVT, Node->getOperand(0));
     Op = DAG.getVectorShuffle(ByteVT, DL, Op, DAG.getUNDEF(ByteVT), ShuffleMask);
     return DAG.getNode(ISD::BITCAST, DL, VT, Op);
+  }
+
+  // If we have the appropriate vector bit operations, it is better to use them
+  // than unrolling and expanding each component.
+  if (TLI.isOperationLegalOrCustom(ISD::SHL, VT) &&
+      TLI.isOperationLegalOrCustom(ISD::SRL, VT) &&
+      TLI.isOperationLegalOrCustomOrPromote(ISD::AND, VT) &&
+      TLI.isOperationLegalOrCustomOrPromote(ISD::OR, VT))
+    return TLI.expandBSWAP(Node, DAG);
+
+  // Otherwise unroll.
+  return DAG.UnrollVectorOp(Node);
 }
 
 void VectorLegalizer::ExpandBITREVERSE(SDNode *Node,