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/tree/updater_approx.cc

@@ -264,7 +264,7 @@ class GlobalApproxUpdater : public TreeUpdater {
 
  public:
   explicit GlobalApproxUpdater(GenericParameter const *ctx, ObjInfo task)
-      : task_{task}, TreeUpdater(ctx) {
+      : TreeUpdater(ctx), task_{task} {
     monitor_.Init(__func__);
   }
 

src/tree/updater_quantile_hist.cc

@@ -355,11 +355,11 @@ void HistRowPartitioner::FindSplitConditions(const std::vector<CPUExpandEntry> &
     const bst_float split_pt = tree[nid].SplitCond();
     const uint32_t lower_bound = gmat.cut.Ptrs()[fid];
     const uint32_t upper_bound = gmat.cut.Ptrs()[fid + 1];
-    int32_t split_cond = -1;
+    bst_bin_t split_cond = -1;
     // convert floating-point split_pt into corresponding bin_id
     // split_cond = -1 indicates that split_pt is less than all known cut points
     CHECK_LT(upper_bound, static_cast<uint32_t>(std::numeric_limits<int32_t>::max()));
-    for (uint32_t bound = lower_bound; bound < upper_bound; ++bound) {
+    for (auto bound = lower_bound; bound < upper_bound; ++bound) {
       if (split_pt == gmat.cut.Values()[bound]) {
         split_cond = static_cast<int32_t>(bound);
       }

src/tree/updater_quantile_hist.h

@@ -324,7 +324,7 @@ class QuantileHistMaker: public TreeUpdater {
     std::unique_ptr<HistogramBuilder<CPUExpandEntry>> histogram_builder_;
     ObjInfo task_;
     // Context for number of threads
-    GenericParameter const* ctx_;
+    Context const* ctx_;
 
     std::unique_ptr<common::Monitor> monitor_;
   };