pgsql: Clean up bugs in clause_selectivity() cleanup.

Clean up bugs in clause_selectivity() cleanup.

Commit ac2b09508 was not terribly carefully reviewed. Band-aid it to
not fail on non-RestrictInfo input, per report from Andreas Seltenreich.
Also make it do something more reasonable with variable-free clauses,
and improve nearby comments.

Discussion: https://postgr.es/m/87inmf5rdx.fsf@credativ.de

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/eef8c0069e4d5eea2e52965ce3eb018b5a594fd6

Modified Files
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src/backend/optimizer/path/clausesel.c | 81 ++++++++++++++++++----------------
1 file changed, 42 insertions(+), 39 deletions(-)