Fix grammar's AND/OR flattening to work with operator_precedence_warning.
Fix grammar's AND/OR flattening to work with operator_precedence_warning.

It'd be good for "(x AND y) AND z" to produce a three-child AND node
whether or not operator_precedence_warning is on, but that failed to
happen when it's on because makeAndExpr() didn't look through the added
AEXPR_PAREN node.  This has no effect on generated plans because prepqual.c
would flatten the AND nest anyway; but it does affect the number of parens
printed in ruleutils.c, for example.  I'd already fixed some similar
hazards in parse_expr.c in commit abb164655, but didn't think to search
gram.y for problems of this ilk.  Per gripe from Jean-Pierre Pelletier.

Report: <e560593e27f1013eb66a81a344e686601a615dc1@mail.gmail.com>

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