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