[RISC-V] fmv.s/fmv.d should be as cheap as a move

[arichardson]

Summary

Since the canonical floatig-point move is fsgnj rd, rs, rs, we should
handle this case in RISCVInstrInfo::isAsCheapAsAMove().

[lenary]

LGTM!

llvm/lib/Target/RISCV/RISCVInstrInfo.cpp

unsigned RISCVInstrInfo::getInstSizeInBytes(const MachineInstr &MI) const {
   }
 }
 
 bool RISCVInstrInfo::isAsCheapAsAMove(const MachineInstr &MI) const {
   const unsigned Opcode = MI.getOpcode();
   switch (Opcode) {
   default:
     break;
+  case RISCV::FSGNJ_D:
+  case RISCV::FSGNJ_S:
+    // The canonical floatig-point move is fsgnj rd, rs, rs.
+    return MI.getOperand(1).isReg() && MI.getOperand(2).isReg() &&
+           MI.getOperand(1).getReg() == MI.getOperand(2).getReg();
   case RISCV::ADDI:
   case RISCV::ORI:
   case RISCV::XORI:
     return (MI.getOperand(1).isReg() &&
             MI.getOperand(1).getReg() == RISCV::X0) ||
            (MI.getOperand(2).isImm() && MI.getOperand(2).getImm() == 0);
   }
   return MI.isAsCheapAsAMove();