8007763: Refactoring: split up compute_generation_free_space() into two functions for class PSAdaptiveSizePolicy
Summary: split up compute_generation_free_space() into two functions: compute_eden_space_size() + compute_old_gen_free_space(), each of which (if needed) can be reused without executing an overhead of the other.
Reviewed-by: jmasa, tschatzl
Contributed-by: Ntamao <tao.mao@oracle.com>