Clean up training code. (#3825)

* Remove GHistRow, GHistEntry, GHistIndexRow.
* Remove kSimpleStats.
* Remove CheckInfo, SetLeafVec in GradStats and in SKStats.
* Clean up the GradStats.
* Cleanup calcgain.
* Move LossChangeMissing out of common.
* Remove [] operator from GHistIndexBlock.

src/common/transform.h

@@ -58,12 +58,12 @@ class Transform {
    public:
     Evaluator(Functor func, Range range, GPUSet devices, bool reshard) :
         func_(func), range_{std::move(range)},
-        distribution_{std::move(GPUDistribution::Block(devices))},
-        reshard_{reshard} {}
+        reshard_{reshard},
+        distribution_{std::move(GPUDistribution::Block(devices))} {}
     Evaluator(Functor func, Range range, GPUDistribution dist,
               bool reshard) :
-        func_(func), range_{std::move(range)}, distribution_{std::move(dist)},
-        reshard_{reshard} {}
+        func_(func), range_{std::move(range)}, reshard_{reshard},
+        distribution_{std::move(dist)} {}
 
     /*!
      * \brief Evaluate the functor with input pointers to HostDeviceVector.
@@ -159,7 +159,7 @@ class Transform {
 
     template <typename... HDV>
     void LaunchCPU(Functor func, HDV*... vectors) const {
-      auto end = *(range_.end());
+      omp_ulong end = static_cast<omp_ulong>(*(range_.end()));
 #pragma omp parallel for schedule(static)
       for (omp_ulong idx = 0; idx < end; ++idx) {
         func(idx, UnpackHDV(vectors)...);