Evaluate expression at planning time for two more cases

Hi,

In good written query IS NULL and IS NOT NULL check on primary and non null
constraints columns should not happen but if it is mentioned PostgreSQL
have to be smart enough for not checking every return result about null
value on primary key column. Instead it can be evaluate its truth value and
set the result only once. The attached patch evaluate and set the truth
value for null and not null check on primary column on planning time if the
relation attribute is not mention on nullable side of outer join.

Thought?

regards

Surafel