pg_stat_statements: Fix crash in list squashing with Vars

pg_stat_statements: Fix crash in list squashing with Vars

When IN/ANY clauses contain both constants and variable expressions, the
optimizer transforms them into separate structures: constants become
an array expression while variables become individual OR conditions.

This transformation was creating an overlap with the token locations,
causing pg_stat_statements query normalization to crash because it
could not calculate the amount of bytes remaining to write for the
normalized query.

This commit disables squashing for mixed IN list expressions when
constructing a scalar array op, by setting list_start and list_end
to -1 when both variables and non-variables are present.  Some
regression tests are added to PGSS to verify these patterns.

Author: Sami Imseih <samimseih@gmail.com>
Reviewed-by: Dmitry Dolgov <9erthalion6@gmail.com>
Discussion: https://postgr.es/m/CAA5RZ0ts9qiONnHjjHxPxtePs22GBo4d3jZ_s2BQC59AN7XbAA@mail.gmail.com
Backpatch-through: 18