Notice that heap page has dead items during VACUUM.

 Notice that heap page has dead items during VACUUM.
 
 Consistently set a flag variable that tracks whether the current heap
 page has a dead item during lazy vacuum's heap scan.  We missed the
 common case where there is an preexisting (or even a new) LP_DEAD heap
 line pointer.
 
 Also make it clear that the variable might be affected by an existing
 line pointer, say from an earlier opportunistic pruning operation.  This
 distinction is important because it's the main reason why we can't just
 use the nearby tups_vacuumed variable instead.
 
 No backpatch.  In theory failing to set the page level flag variable had
 no consequences.  Currently it is only used to defensively check if a
 page marked all visible has dead items, which should never happen anyway
 (if it does then the table must be corrupt).
 
 Author: Masahiko Sawada <sawada.mshk@gmail.com>
 Diagnosed-By: Masahiko Sawada <sawada.mshk@gmail.com>
 Discussion: https://postgr.es/m/CAD21AoAtZb4+HJT_8RoOXvu4HM-Zd4HKS3YSMCH6+-W=bDyh-w@mail.gmail.com