btr_cur_compress_if_useful(): Replace if () return(); return() with return.

page_rec_get_next_low(): New function.

page_rec_get_prev(): Invoke page_is_comp() outside the loop.
Replace page_rec_get_next() with loop-specific instances of
page_rec_get_next_low().

page_copy_rec_list_end(): Add some debug assertions.