Comments in IndexBuildHeapScan describe the indexing of recently-dead

Comments in IndexBuildHeapScan describe the indexing of recently-dead
tuples as needed "to keep VACUUM from complaining", but actually there is
a more compelling reason to do it: failure to do so violates MVCC semantics.
This is because a pre-existing serializable transaction might try to use
the index after we finish (re)building it, and it might fail to find tuples
it should be able to see.  We got this mostly right, but not in the case
of partial indexes: the code mistakenly discarded recently-dead tuples for
partial indexes.  Fix that, and adjust the comments.