[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

mlir/lib/Dialect/SparseTensor/Transforms/SparseTensorRewriting.cpp

@@ -839,7 +839,7 @@ struct ConvertRewriter : public OpRewritePattern<ConvertOp> {
       } else {
         // Gather the indices-arrays in the dst tensor storage order.
         SmallVector<Value> xs(rank, Value());
-        for (uint64_t i = 0; i < rank; i++) {
+        for (int64_t i = 0; i < rank; i++) {
           uint64_t orgDim = toOrigDim(encSrc, i);
           xs[toStoredDim(encDst, orgDim)] =
               genToIndices(rewriter, loc, src, i, /*cooStart=*/0);