[PATCH] Optimize IS DISTINCT FROM NULL => IS NOT NULL

Hi list,

This patch enables a simple optimization in
eval_const_expressions_mutator. If we know that one argument to
DistinctExpr is NULL then we can optimize it to a NullTest, which can
be an indexable expression.

For example the query:
EXPLAIN (costs off) SELECT * FROM foo WHERE j IS NOT DISTINCT FROM NULL;

Old behavior:
Seq Scan on foo
  Filter: (NOT (j IS DISTINCT FROM NULL::integer))

New behavior:
Index Scan using foo_j_idx on foo
  Index Cond: (j IS NULL)

Regards,
Marti