Fix const-simplification for constraints and stats

Fix const-simplification for constraints and stats

Constraint expressions and statistics expressions loaded from the
system catalogs need to be run through const-simplification, because
the planner will be comparing them to similarly-processed qual
clauses.  Without this step, the planner may fail to detect valid
matches.

Currently, NullTest clauses in these expressions may not be reduced
correctly during const-simplification.  This happens because their Var
nodes do not yet have the correct varno when eval_const_expressions is
applied.  Since eval_const_expressions relies on varno to reduce
NullTest quals, incorrect varno can cause problems.

Additionally, for statistics expressions, eval_const_expressions is
called with root set to NULL, which also inhibits NullTest reduction.

This patch fixes the issue by ensuring that Vars are updated to have
the correct varno before const-simplification, and that a valid root
is passed to eval_const_expressions when needed.

Author: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/19007-4cc6e252ed8aa54a@postgresql.org