[LoongArch] Fix -Wunused-variable in LoongArchExpandPseudoInsts.cpp (NFC)

llvm-project/llvm/lib/Target/LoongArch/LoongArchExpandPseudoInsts.cpp:480:20:
 error: unused variable 'MF' [-Werror,-Wunused-variable]
  MachineFunction *MF = MBB.getParent();
                   ^
1 error generated.