[mlir] Fix two build warnings (NFC)

/data/llvm-project/mlir/lib/Dialect/Tensor/Utils/Utils.cpp:62:11: error: comparison of integers of different signs: 'int64_t' (aka 'long') and 'size_t' (aka 'unsigned long') [-Werror,-Wsign-compare]
  if (dim >= shape.size())
      ~~~ ^  ~~~~~~~~~~~~
1 error generated.

/data/llvm-project/mlir/lib/Dialect/Tensor/IR/TensorTilingInterfaceImpl.cpp:484:8: error: unused variable 'appendIndex' [-Werror,-Wunused-variable]
  auto appendIndex = [&](Value val, SmallVector<Value> &dynIndices,
       ^
1 error generated.