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

[DamonFool]

Summary

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.

[DamonFool]

Thanks @MaskRay for your review.

I don’t have commit access, can you land this patch for me?
Please use “Jie Fu [e-mail]” to commit the change.
Thanks.

[DamonFool]

Thanks @jpienaar for your help.

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

if (!isAllDimOrdered(srcTp)) {
                             /*withLayout=*/false);
         Value xs = rewriter.create<ToIndicesBufferOp>(loc, indTp, src);
         rewriter.create<SortCooOp>(loc, nnz, xs, ValueRange{y},
                                    rewriter.getIndexAttr(rank),
                                    rewriter.getIndexAttr(0));
       } 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);
         }
         rewriter.create<SortOp>(loc, nnz, xs, ValueRange{y});
       }
     }