Fix operator typo in tablecmds.c

Fix operator typo in tablecmds.c

A bitwise operator was getting used on two bools in
ATAddCheckConstraint() to track if constraints should be merged or not
with the existing ones of a relation, though obviously this should use
a boolean OR operator.  This led to the same result, but let's be
clean.

Oversight in 074c5cf.

Author: Ranier Vilela
Reviewed-by: Justin Pryzby
Discussion: https://postgr.es/m/CAEudQAp2R2fbbi0OHHhv_n4=Ch0t1VtjObR9YMqtGKHJ+faUFQ@mail.gmail.com