Ignore nullingrels when looking up statistics

Ignore nullingrels when looking up statistics

When looking up statistical data about an expression, we do not need
to concern ourselves with the outer joins that could null the
Vars/PHVs contained in the expression.  Accounting for nullingrels in
the expression could cause estimate_num_groups to count the same Var
multiple times if it's marked with different nullingrels.  This is
incorrect, and could lead to "ERROR:  corrupt MVNDistinct entry" when
searching for multivariate n-distinct.

Furthermore, the nullingrels could prevent us from matching an
expression to expressional index columns or to the expressions in
extended statistics, leading to inaccurate estimates.

To fix, strip out all the nullingrels from the expression before we
look up statistical data about it.  There is one ensuing plan change
in the regression tests, but it looks reasonable and does not
compromise its original purpose.

This patch could result in plan changes, but it fixes an actual bug,
so back-patch to v16 where the outer-join-aware-Var infrastructure was
introduced.

Author: Richard Guo
Discussion: https://postgr.es/m/CAMbWs4-2Z4k+nFTiZe0Qbu5n8juUWenDAtMzi98bAZQtwHx0-w@mail.gmail.com