ANALYZE ROOTPARTITION always create stats for root partition
ANALYZE ROOTPARTITION always create stats for root partition

Currently, `ANALYZE ROOTPARTITION` merge statistics when
`optimizer_analyze_root_partition` is set to on and NOT* when it is off.
This is an incorrect behavior, it should merge stats if possible
immaterial of the GUC.

This commit fixes this issue.

ashwinstar/gpdb,adam8157/gpdb,jmcatamney/gpdb,adam8157/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,lisakowen/gpdb,jmcatamney/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,ashwinstar/gpdb,xinzweb/gpdb,lisakowen/gpdb,ashwinstar/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,50wu/gpdb,jmcatamney/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,ashwinstar/gpdb,lisakowen/gpdb,ashwinstar/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,lisakowen/gpdb,50wu/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,adam8157/gpdb