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

/llvm-project/llvm/lib/Target/AMDGPU/SIISelLowering.cpp:11257:7: error: unused variable 'VT' [-Werror,-Wunused-variable]
  EVT VT = N->getValueType(0);
      ^
1 error generated.