Fix and cleanup for column matrix. (#7901)

* Fix missed type dispatching for dense columns with missing values.
* Code cleanup to reduce special cases.
* Reduce memory usage.

src/objective/adaptive.cc

@@ -28,7 +28,7 @@ void EncodeTreeLeafHost(RegTree const& tree, std::vector<bst_node_t> const& posi
     sorted_pos[i] = position[ridx[i]];
   }
   // find the first non-sampled row
-  auto begin_pos =
+  size_t begin_pos =
       std::distance(sorted_pos.cbegin(), std::find_if(sorted_pos.cbegin(), sorted_pos.cend(),
                                                       [](bst_node_t nidx) { return nidx >= 0; }));
   CHECK_LE(begin_pos, sorted_pos.size());