[DAG] Remove unused variable 'VT' in DAGCombiner.cpp (NFC)

/llvm-project/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:26896:7: error: unused variable 'VT' [-Werror,-Wunused-variable]
  EVT VT = N->getValueType(0);
      ^
1 error generated.