[RISCV] Remove FrameIndex case in lui+addi MacroFusion (#68701)

If the first operand of ADDI is a frame index, then it won't have data
dependency of predecessor LUI. So it is impossible to do the DAG
mutation in these two instructions.

Author: wangpc-pp

llvm/lib/Target/RISCV/RISCVMacroFusion.cpp

@@ -35,10 +35,6 @@ static bool isLUIADDI(const MachineInstr *FirstMI,
   if (FirstMI->getOpcode() != RISCV::LUI)
     return false;
 
-  // The first operand of ADDI might be a frame index.
-  if (!SecondMI.getOperand(1).isReg())
-    return false;
-
   Register FirstDest = FirstMI->getOperand(0).getReg();
 
   // Destination of LUI should be the ADDI(W) source register.