[AMDGPU] Fix -Wunused-function in GCNDPPCombine.cpp (NFC)

/llvm-project/llvm/lib/Target/AMDGPU/GCNDPPCombine.cpp:194:17: error: unused function 'getOperandSize' [-Werror,-Wunused-function]
static unsigned getOperandSize(MachineInstr &MI, unsigned Idx,
                ^
1 error generated.