[mlir] Fix -Wsign-compare in SparseTensorRewriting.cpp and Sparsification.cpp (NFC)

/home/jiefu/llvm-project/mlir/lib/Dialect/SparseTensor/Transforms/Sparsification.cpp:279:33: error: comparison of integers of different signs: 'int64_t' (aka 'long') and 'const mlir::sparse_tensor::Level' (aka 'const unsigned long') [-Werror,-Wsign-compare]
    assert(env.op().getRank(&t) == lvlRank);
           ~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~
/usr/include/assert.h:93:27: note: expanded from macro 'assert'
     (static_cast <bool> (expr)                                         \
                          ^~~~
1 error generated.

/home/jiefu/llvm-project/mlir/lib/Dialect/SparseTensor/Transforms/SparseTensorRewriting.cpp:788:29: error: comparison of integers of different signs: 'int64_t' (aka 'long') and 'const mlir::sparse_tensor::Dimension' (aka 'const unsigned long') [-Werror,-Wsign-compare]
    assert(srcRTT.getRank() == dimRank);
           ~~~~~~~~~~~~~~~~ ^  ~~~~~~~
/usr/include/assert.h:93:27: note: expanded from macro 'assert'
     (static_cast <bool> (expr)                                         \
                          ^~~~
/home/jiefu/llvm-project/mlir/lib/Dialect/SparseTensor/Transforms/SparseTensorRewriting.cpp:810:31: error: comparison of integers of different signs: 'int64_t' (aka 'long') and 'const mlir::sparse_tensor::Dimension' (aka 'const unsigned long') [-Werror,-Wsign-compare]
      assert(srcRTT.getRank() == dimRank);
             ~~~~~~~~~~~~~~~~ ^  ~~~~~~~
/usr/include/assert.h:93:27: note: expanded from macro 'assert'
     (static_cast <bool> (expr)                                         \
                          ^~~~
2 errors generated.