Update JIT tuple deforming code for virtual generated columns

Update JIT tuple deforming code for virtual generated columns

The JIT deforming code contains an optimization that determines which
columns are guaranteed to exist in the tuple.  That's used to allow
skipping of reading the tuple's natts when the code only needs to deform
attributes that are guaranteed to always exist in all tuples.  83ea6c540
missed updating this code to account for VIRTUAL generated columns.
These are stored as NULLs in the tuple, but may be defined as NOT NULL.
This could result in the code thinking more columns are guaranteed to
exist than actually do.

Author: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Backpatch-through: 18
Discussion: https://postgr.es/m/1151393.1781734980@sss.pgh.pa.us