[mlir] Fix build error due to -Wsign-compare after revision D140871

This patch fixes build failure due to -Wsign-compare in sparse2SparseRewrite(...) after https://reviews.llvm.org/D140871.

```
llvm-project/mlir/lib/Dialect/SparseTensor/Transforms/SparseTensorRewriting.cpp:842:32: error: comparison of integers of different signs: 'uint64_t' (aka 'unsigned long') and 'int64_t' (aka 'long') [-Werror,-Wsign-compare]
        for (uint64_t i = 0; i < rank; i++) {
                             ~ ^ ~~~~
1 error generated.
```

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D141104