This is important so JIT compilation knows what kind of tuple slot the
deforming routine can expect. There's also optimization potential for
expression initialization without JIT compilation. It e.g. seems
plausible to elide EEOP_*_FETCHSOME ops entirely when dealing with
virtual slots.

Author: Andres Freund
Discussion: https://postgr.es/m/20181105210039.hh4vvi4vwoq5ba2q@alap3.anarazel.de