pgsql: Fix oversight in outer join removal.

Fix oversight in outer join removal.

A placeholder that references the outer join's relid in ph_eval_at
is logically "above" the join, and therefore we can't remove its
PlaceHolderInfo: it might still be used somewhere in the query.

This was not an issue pre-v16 because we failed to remove the join
at all in such cases. The new outer-join-aware-Var infrastructure
permits deducing that it's okay to remove the join, but then we
have to clean up correctly afterwards.

Report and fix by Richard Guo

Discussion: https://postgr.es/m/CAMbWs4_tuVn9EwwMcggGiZJWWstdXX_ci8FeEU17vs+4nLgw3w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9a2dbc614e6e47da3c49daacec106da32eba9467

Modified Files
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src/backend/optimizer/plan/analyzejoins.c | 3 +-
src/test/regress/expected/join.out | 57 +++++++++++++++++++++++++++++++
src/test/regress/sql/join.sql | 34 ++++++++++++++++++
3 files changed, 93 insertions(+), 1 deletion(-)