Avoid O(N^2) cost when pulling up lots of UNION ALL subqueries.

perform_pullup_replace_vars() knows how to scan the whole parent
query tree when we are replacing Vars during a subquery flattening
operation.  However, for the specific case of flattening a
UNION ALL leaf query, that's mostly wasted work: the only place
where relevant Vars could exist is in the AppendRelInfo that we
just made for this leaf.  Teaching perform_pullup_replace_vars()
to just deal with that and exit is worthwhile because, if we have
N such subqueries to pull up, we were spending O(N^2) work uselessly
mutating the AppendRelInfos for all the other subqueries.

While we're at it, avoid calling substitute_phv_relids if there are no
PlaceHolderVars, and remove an obsolete check of parse->hasSubLinks.

Andrey Lepikhov and Tom Lane

Discussion: https://postgr.es/m/703c09a2-08f3-d2ec-b33d-dbecd62428b8@postgrespro.ru

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