pgsql: Add regression test for various power expressions with a zero

Log Message:
-----------
Add regression test for various power expressions with a zero base, and
adjust source code to be more modular.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        numeric.c (r1.111 -> r1.112)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/numeric.c?r1=1.111&r2=1.112)
    pgsql/src/test/regress/expected:
        float8.out (r1.25 -> r1.26)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/float8.out?r1=1.25&r2=1.26)
    pgsql/src/test/regress/sql:
        float8.sql (r1.15 -> r1.16)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/float8.sql?r1=1.15&r2=1.16)