pgsql: Fix Assert failure induced by commit 215b43cdc.

Fix Assert failure induced by commit 215b43cdc.

I'd somehow talked myself into believing that set_append_rel_size
doesn't need to worry about getting back an AND clause when it applies
eval_const_expressions to the result of adjust_appendrel_attrs (that is,
transposing the appendrel parent's restriction clauses for one child).
But that is nonsense, and Andreas Seltenreich's fuzz tester soon
turned up a counterexample. Put back the make_ands_implicit step
that was there before, and add a regression test covering the case.

Report: https://postgr.es/m/878tq6vja6.fsf@ansel.ydns.eu

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/d479e37e3d20efad8b178e0f1e468c086a7519a8

Modified Files
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src/backend/optimizer/path/allpaths.c | 43 ++++++++++++++++++++---------------
src/test/regress/expected/union.out | 30 ++++++++++++++++++++++++
src/test/regress/sql/union.sql | 13 +++++++++++
3 files changed, 68 insertions(+), 18 deletions(-)