[RISCV][NFC] Remove dead code.

[fakepaper56]

Summary

All ISD::BSWAP nodes are not customized lowered in RISC-V now, so the patch
removed dead code for ISD::BSWAP in LowerOperation.

[craig.topper]

LGTM

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

case ISD::BITCAST: {
     return SDValue();
   }
   case ISD::INTRINSIC_WO_CHAIN:
     return LowerINTRINSIC_WO_CHAIN(Op, DAG);
   case ISD::INTRINSIC_W_CHAIN:
     return LowerINTRINSIC_W_CHAIN(Op, DAG);
   case ISD::INTRINSIC_VOID:
     return LowerINTRINSIC_VOID(Op, DAG);
-  case ISD::BSWAP:
   case ISD::BITREVERSE: {
     MVT VT = Op.getSimpleValueType();
     SDLoc DL(Op);
     assert(Subtarget.hasStdExtZbkb() && "Unexpected custom legalization");
     assert(Op.getOpcode() == ISD::BITREVERSE && "Unexpected opcode");
     // Expand bitreverse to a bswap(rev8) followed by brev8.
     SDValue BSwap = DAG.getNode(ISD::BSWAP, DL, VT, Op.getOperand(0));
     return DAG.getNode(RISCVISD::BREV8, DL, VT, BSwap);