Standardize on the assumption that the arguments of a RowExpr correspond

Standardize on the assumption that the arguments of a RowExpr correspond
to the physical layout of the rowtype, ie, there are dummy arguments
corresponding to any dropped columns in the rowtype.  We formerly had a
couple of places that did it this way and several others that did not.
Fixes Gaetano Mendola's "cache lookup failed for type 0" bug of 5-Aug.