pgsql: Fix oversight in new code for printing rangetable aliases.

Fix oversight in new code for printing rangetable aliases.

In commit 11e131854f8231a21613f834c40fe9d046926387, I missed the case of
a CTE RTE that doesn't have a user-defined alias, but does have an
alias assigned by set_rtable_names(). Per report from Peter Eisentraut.

While at it, refactor slightly to reduce code duplication.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8b728e5c6e0ce6b6d6f54b92b390f14aa1aca6db

Modified Files
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src/backend/utils/adt/ruleutils.c | 31 +++++++++++++++++++++----------
src/test/regress/expected/with.out | 24 ++++++++++++++++++++++++
src/test/regress/sql/with.sql | 11 +++++++++++
3 files changed, 56 insertions(+), 10 deletions(-)