[AMDGPU] Fix -Wunused-variable in SIFrameLowering.cpp (NFC)

/llvm-project/llvm/lib/Target/AMDGPU/SIFrameLowering.cpp:1829:8: error: unused variable 'IsChainFunction' [-Werror,-Wunused-variable]
  bool IsChainFunction = MF.getInfo<SIMachineFunctionInfo>()->isChainFunction();
       ^
1 error generated.