pgsql: In plpgsql, don't try to convert int2vector or oidvector to expa

In plpgsql, don't try to convert int2vector or oidvector to expanded array.

These types are storage-compatible with real arrays, but they don't support
toasting, so of course they can't support expansion either.

Per bug #14289 from Michael Overmeyer.  Back-patch to 9.5 where expanded
arrays were introduced.

Report: <20160818174414.1529.37913@wrigleys.postgresql.org>

Branch
------
REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/c81c71d8846fa1667e67b45559ce4c72a8e076d9

Modified Files
--------------
src/include/utils/array.h    | 2 +-
src/pl/plpgsql/src/pl_comp.c | 9 +++++++--
2 files changed, 8 insertions(+), 3 deletions(-)