Further optimisations for gpu_hist. (#4283)

- Fuse final update position functions into a single more efficient kernel

- Refactor gpu_hist with a more explicit ellpack  matrix representation

tests/cpp/helpers.h

@@ -26,7 +26,7 @@
 
 bool FileExists(const std::string& filename);
 
-long GetFileSize(const std::string& filename);
+int64_t GetFileSize(const std::string& filename);
 
 void CreateSimpleTestData(const std::string& filename);