[mlir] Fix a warning

This patch fixes:

  mlir/lib/Dialect/Tensor/IR/TensorOps.cpp:1680:37: error: comparison
  of integers of different signs: 'int' and 'uint64_t' (aka 'unsigned
  long') [-Werror,-Wsign-compare]

Author: kazutakahirata

mlir/lib/Dialect/Tensor/IR/TensorOps.cpp

@@ -1676,7 +1676,7 @@ class InsertOpConstantFold final : public OpRewritePattern<InsertOp> {
         auto flattenedIndex = sourceAttr.getFlattenedIndex(indices);
         std::vector<Attribute> updatedValues;
         updatedValues.reserve(sourceAttr.getNumElements());
-        for (auto i = 0; i < sourceAttr.getNumElements(); ++i) {
+        for (unsigned i = 0; i < sourceAttr.getNumElements(); ++i) {
           updatedValues.push_back(i == flattenedIndex ? scalarAttr
                                                       : sourceValues[i]);
         }