select constant exprs should have the  same constraints as select instructions, notably, they should support vectors and aggregates.
select constant exprs should have the  same constraints as select instructions,
notably, they should support vectors and aggregates.


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@61462 91177308-0d34-0410-b5e6-96231b3b80d8

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