Fix a pair of related issues with estimation of inequalities that involve binary-compatible relabeling of one or both operands.  examine_variable should avoid stripping RelabelType from non-variable expressions, so that they will continue to have the correct type; and convert_to_scalar should just use that type and ignore the other input type.  This isn't perfect but it beats failing entirely.  Per example from Michael Fuhr.
Fix a pair of related issues with estimation of inequalities that involve
binary-compatible relabeling of one or both operands.  examine_variable
should avoid stripping RelabelType from non-variable expressions, so that
they will continue to have the correct type; and convert_to_scalar should
just use that type and ignore the other input type.  This isn't perfect
but it beats failing entirely.  Per example from Michael Fuhr.

0x0FFF/gpdb,tpostgres-projects/tPostgres,janebeckman/gpdb,tangp3/gpdb,rvs/gpdb,tangp3/gpdb,Chibin/gpdb,cjcjameson/gpdb,arcivanov/postgres-xl,postmind-net/postgres-xl,xinzweb/gpdb,xinzweb/gpdb,xuegang/gpdb,chrishajas/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,lpetrov-pivotal/gpdb,rvs/gpdb,postmind-net/postgres-xl,techdragon/Postgres-XL,ahachete/gpdb,rubikloud/gpdb,janebeckman/gpdb,zaksoup/gpdb,xuegang/gpdb,greenplum-db/gpdb,kaknikhil/gpdb,xuegang/gpdb,arcivanov/postgres-xl,CraigHarris/gpdb,xuegang/gpdb,tangp3/gpdb,0x0FFF/gpdb,lintzc/gpdb,cjcjameson/gpdb,lintzc/gpdb,cjcjameson/gpdb,lintzc/gpdb,jmcatamney/gpdb,Quikling/gpdb,chrishajas/gpdb,tpostgres-projects/tPostgres,kmjungersen/PostgresXL,randomtask1155/gpdb,xuegang/gpdb,edespino/gpdb,rubikloud/gpdb,edespino/gpdb,lisakowen/gpdb,janebeckman/gpdb,0x0FFF/gpdb,royc1/gpdb,Postgres-XL/Postgres-XL,50wu/gpdb,zaksoup/gpdb,Chibin/gpdb,ahachete/gpdb,atris/gpdb,Quikling/gpdb,lpetrov-pivotal/gpdb,lpetrov-pivotal/gpdb,ahachete/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,atris/gpdb,Chibin/gpdb,xinzweb/gpdb,greenplum-db/gpdb,oberstet/postgres-xl,rvs/gpdb,ashwinstar/gpdb,randomtask1155/gpdb,randomtask1155/gpdb,adam8157/gpdb,pavanvd/postgres-xl,ovr/postgres-xl,cjcjameson/gpdb,rubikloud/gpdb,yazun/postgres-xl,zaksoup/gpdb,lpetrov-pivotal/gpdb,edespino/gpdb,rubikloud/gpdb,janebeckman/gpdb,edespino/gpdb,foyzur/gpdb,xinzweb/gpdb,0x0FFF/gpdb,tangp3/gpdb,kaknikhil/gpdb,royc1/gpdb,greenplum-db/gpdb,foyzur/gpdb,rubikloud/gpdb,0x0FFF/gpdb,CraigHarris/gpdb,techdragon/Postgres-XL,adam8157/gpdb,cjcjameson/gpdb,pavanvd/postgres-xl,snaga/postgres-xl,rubikloud/gpdb,CraigHarris/gpdb,ashwinstar/gpdb,rvs/gpdb,jmcatamney/gpdb,lisakowen/gpdb,adam8157/gpdb,Quikling/gpdb,Chibin/gpdb,kaknikhil/gpdb,royc1/gpdb,rvs/gpdb,arcivanov/postgres-xl,ahachete/gpdb,rvs/gpdb,ahachete/gpdb,snaga/postgres-xl,ahachete/gpdb,ovr/postgres-xl,arcivanov/postgres-xl,ovr/postgres-xl,yuanzhao/gpdb,50wu/gpdb,xuegang/gpdb,zaksoup/gpdb,kaknikhil/gpdb,royc1/gpdb,chrishajas/gpdb,lisakowen/gpdb,randomtask1155/gpdb,kaknikhil/gpdb,pavanvd/postgres-xl,xinzweb/gpdb,50wu/gpdb,CraigHarris/gpdb,lpetrov-pivotal/gpdb,lpetrov-pivotal/gpdb,xinzweb/gpdb,zeroae/postgres-xl,foyzur/gpdb,lisakowen/gpdb,kaknikhil/gpdb,ashwinstar/gpdb,arcivanov/postgres-xl,rvs/gpdb,kmjungersen/PostgresXL,lintzc/gpdb,cjcjameson/gpdb,kaknikhil/gpdb,CraigHarris/gpdb,kmjungersen/PostgresXL,oberstet/postgres-xl,kaknikhil/gpdb,Chibin/gpdb,chrishajas/gpdb,tangp3/gpdb,postmind-net/postgres-xl,adam8157/gpdb,techdragon/Postgres-XL,foyzur/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,Quikling/gpdb,zeroae/postgres-xl,yuanzhao/gpdb,zaksoup/gpdb,snaga/postgres-xl,chrishajas/gpdb,lpetrov-pivotal/gpdb,cjcjameson/gpdb,edespino/gpdb,foyzur/gpdb,ashwinstar/gpdb,postmind-net/postgres-xl,foyzur/gpdb,ashwinstar/gpdb,arcivanov/postgres-xl,pavanvd/postgres-xl,atris/gpdb,randomtask1155/gpdb,jmcatamney/gpdb,cjcjameson/gpdb,Quikling/gpdb,Quikling/gpdb,randomtask1155/gpdb,zaksoup/gpdb,janebeckman/gpdb,zaksoup/gpdb,CraigHarris/gpdb,tpostgres-projects/tPostgres,jmcatamney/gpdb,snaga/postgres-xl,kaknikhil/gpdb,tangp3/gpdb,chrishajas/gpdb,tpostgres-projects/tPostgres,0x0FFF/gpdb,0x0FFF/gpdb,yuanzhao/gpdb,yazun/postgres-xl,0x0FFF/gpdb,lintzc/gpdb,atris/gpdb,xuegang/gpdb,lpetrov-pivotal/gpdb,Postgres-XL/Postgres-XL,Chibin/gpdb,postmind-net/postgres-xl,zaksoup/gpdb,Chibin/gpdb,CraigHarris/gpdb,Quikling/gpdb,janebeckman/gpdb,ashwinstar/gpdb,zeroae/postgres-xl,tangp3/gpdb,Quikling/gpdb,yuanzhao/gpdb,tangp3/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,randomtask1155/gpdb,rubikloud/gpdb,xinzweb/gpdb,royc1/gpdb,cjcjameson/gpdb,edespino/gpdb,tpostgres-projects/tPostgres,ahachete/gpdb,janebeckman/gpdb,atris/gpdb,xuegang/gpdb,chrishajas/gpdb,cjcjameson/gpdb,rvs/gpdb,kmjungersen/PostgresXL,atris/gpdb,rvs/gpdb,lintzc/gpdb,oberstet/postgres-xl,ovr/postgres-xl,CraigHarris/gpdb,lisakowen/gpdb,techdragon/Postgres-XL,pavanvd/postgres-xl,adam8157/gpdb,yazun/postgres-xl,ovr/postgres-xl,chrishajas/gpdb,lintzc/gpdb,techdragon/Postgres-XL,Chibin/gpdb,50wu/gpdb,xuegang/gpdb,lisakowen/gpdb,royc1/gpdb,jmcatamney/gpdb,Postgres-XL/Postgres-XL,foyzur/gpdb,janebeckman/gpdb,50wu/gpdb,yazun/postgres-xl,yazun/postgres-xl,edespino/gpdb,lisakowen/gpdb,edespino/gpdb,yuanzhao/gpdb,Quikling/gpdb,royc1/gpdb,foyzur/gpdb,janebeckman/gpdb,Quikling/gpdb,lintzc/gpdb,rubikloud/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,kmjungersen/PostgresXL,Chibin/gpdb,yuanzhao/gpdb,Postgres-XL/Postgres-XL,ahachete/gpdb,kaknikhil/gpdb,rvs/gpdb,royc1/gpdb,lisakowen/gpdb,snaga/postgres-xl,zeroae/postgres-xl,50wu/gpdb,atris/gpdb,zeroae/postgres-xl,oberstet/postgres-xl,50wu/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,edespino/gpdb,janebeckman/gpdb,oberstet/postgres-xl,CraigHarris/gpdb,Chibin/gpdb,ashwinstar/gpdb,randomtask1155/gpdb,jmcatamney/gpdb,lintzc/gpdb,Postgres-XL/Postgres-XL,yuanzhao/gpdb,atris/gpdb,edespino/gpdb