pgsql: Add basic regression tests for semi/antijoin recognition.

Add basic regression tests for semi/antijoin recognition.

Add some simple tests that the planner recognizes all the
standard idioms for SEMI and ANTI joins.  Failure to optimize
in this way won't necessarily cause any visible change in
query results, so check the plans.  We had no similar coverage
before, at least for some variants of antijoin, as noted by
Richard Guo.

Discussion: https://postgr.es/m/CAMbWs4-mvPPCJ1W6iK6dD5HiNwoJdi6mZp=-7mE8N9Sh+cd0tQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0043aa6b8597809b19c420cb5e635b4d832adccc

Modified Files
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src/test/regress/expected/join.out | 63 ++++++++++++++++++++++++++++++++++++++
src/test/regress/sql/join.sql      | 25 +++++++++++++++
2 files changed, 88 insertions(+)