cache, especially buf_pool->LRU_old and bpage->old.

buf_LRU_old_adjust_len(), buf_LRU_remove_block(): Check that blocks in
buf_pool->LRU_old have the "old" flag set and the blocks preceding
buf_pool->LRU_old have the "old" flag clear.

buf_LRU_add_block_low(), buf_relocate(): Check that buf_pool->LRU_old
is the first block in the LRU list whose "old" flag is set.

buf_LRU_free_block(): When replacing a control block in the LRU list
with a control block for a compressed page, assert that the "old"
flags in the neighboring LRU list entries grow monotonically.

buf_page_set_old(): Assert that the "old" flags in the neighboring LRU
list entries grow monotonically.