[AMDGPU] Remove unused variable 'CNI' in /AMDGPUMachineCFGStructurizer.cpp (NFC)

/Users/jiefu/llvm-project/llvm/lib/Target/AMDGPU/AMDGPUMachineCFGStructurizer.cpp:2603:10: error: variable 'CNI' set but not used [-Werror,-Wunused-but-set-variable]
    auto CNI = CI;
         ^
1 error generated.