pgsql: exec_eval_simple_expr() needs to do CommandCounterIncrement() not

Log Message:
-----------
exec_eval_simple_expr() needs to do CommandCounterIncrement() not just
GetTransactionSnapshot() to ensure ActiveSnapshot advances properly.
Sigh.  Extend regression test so it reveals this error too.

Modified Files:
--------------
    pgsql/src/pl/plpgsql/src:
        pl_exec.c (r1.125 -> r1.126)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.125&r2=1.126)
    pgsql/src/test/regress/expected:
        plpgsql.out (r1.23 -> r1.24)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plpgsql.out.diff?r1=1.23&r2=1.24)
    pgsql/src/test/regress/sql:
        plpgsql.sql (r1.18 -> r1.19)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/plpgsql.sql.diff?r1=1.18&r2=1.19)