The partial paths that get modified may already have been used as
part of a GatherPath which appears in the path list, so modifying
them is not a good idea at this stage - especially because this
code has no check that the PathTarget is in fact parallel-safe.

When partial aggregation is being performed, this is actually
harmless because we'll end up replacing the pathtargets here with
the correct ones within create_grouping_paths().  But if we've got
a query tree containing only scan/join operations then this can
result in incorrectly pushing down parallel-restricted target
list entries.  If those are, for example, references to subqueries,
that can crash the server; but it's wrong in any event.

Amit Kapila