pgsql: Fix incorrect is-this-the-topmost-join tests in parallel plannin

Fix incorrect is-this-the-topmost-join tests in parallel planning.

Two callers of generate_useful_gather_paths were testing the wrong
thing when deciding whether to call that function: they checked for
being at the top of the current join subproblem, rather than being at
the actual top join. This'd result in failing to construct parallel
paths for a sub-join for which they might be useful.

While set_rel_pathlist() isn't actively broken, it seems best to
make its identical-in-intention test for this be like the other two.

This has been wrong all along, but given the lack of field complaints
I'm hesitant to back-patch into stable branches; we usually prefer
to avoid non-bug-fix changes in plan choices in minor releases.
It seems not too late for v15 though.

Richard Guo, reviewed by Antonin Houska and Tom Lane

Discussion: https://postgr.es/m/CAMbWs4-mH8Zf87-w+3P2J=nJB+5OyicO28ia9q_9o=Lamf_VHg@mail.gmail.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a3699c599ced03ace09646b74b8299d8f570ca76

Modified Files
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src/backend/optimizer/geqo/geqo_eval.c | 2 +-
src/backend/optimizer/path/allpaths.c | 9 ++++-----
2 files changed, 5 insertions(+), 6 deletions(-)