Reduce "X = X" to "X IS NOT NULL", if it's easy to do so.

If the operator is a strict btree equality operator, and X isn't volatile,
then the clause must yield true for any non-null value of X, or null if X
is null.  At top level of a WHERE clause, we can ignore the distinction
between false and null results, so it's valid to simplify the clause to
"X IS NOT NULL".  This is a useful improvement mainly because we'll get
a far better selectivity estimate in most cases.

Because such cases seldom arise in well-written queries, it is unappetizing
to expend a lot of planner cycles looking for them ... but it turns out
that there's a place we can shoehorn this in practically for free, because
equivclass.c already has to detect and reject candidate equivalences of the
form X = X.  That doesn't catch every place that it would be valid to
simplify to X IS NOT NULL, but it catches the typical case.  Working harder
doesn't seem justified.

Patch by me, reviewed by Petr Jelinek

Discussion: https://postgr.es/m/CAMjNa7cC4X9YR-vAJS-jSYCajhRDvJQnN7m2sLH1wLh-_Z2bsw@mail.gmail.com

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