Refactor gpu_hist split evaluation (#5610)

* Refactor

* Rewrite evaluate splits

* Add more tests

src/tree/gpu_hist/evaluate_splits.cuh

@@ -0,0 +1,37 @@
+/*!
+ * Copyright 2020 by XGBoost Contributors
+ */
+#ifndef EVALUATE_SPLITS_CUH_
+#define EVALUATE_SPLITS_CUH_
+#include <xgboost/span.h>
+#include "../../data/ellpack_page.cuh"
+#include "../constraints.cuh"
+#include "../updater_gpu_common.cuh"
+
+namespace xgboost {
+namespace tree {
+
+template <typename GradientSumT>
+struct EvaluateSplitInputs {
+  int nidx;
+  GradientSumT parent_sum;
+  GPUTrainingParam param;
+  common::Span<const bst_feature_t> feature_set;
+  common::Span<const uint32_t> feature_segments;
+  common::Span<const float> feature_values;
+  common::Span<const float> min_fvalue;
+  common::Span<const GradientSumT> gradient_histogram;
+  ValueConstraint value_constraint;
+  common::Span<const int> monotonic_constraints;
+};
+template <typename GradientSumT>
+void EvaluateSplits(common::Span<DeviceSplitCandidate> out_splits,
+                    EvaluateSplitInputs<GradientSumT> left,
+                    EvaluateSplitInputs<GradientSumT> right);
+template <typename GradientSumT>
+void EvaluateSingleSplit(common::Span<DeviceSplitCandidate> out_split,
+                         EvaluateSplitInputs<GradientSumT> input);
+}  // namespace tree
+}  // namespace xgboost
+
+#endif  // EVALUATE_SPLITS_CUH_