Clean up bugs in clause_selectivity() cleanup.

Commit ac2b0950 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