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

src/include/optimizer/paths.h

@@ -127,7 +127,8 @@ typedef bool (*ec_matches_callback_type) (PlannerInfo *root,
 										  EquivalenceMember *em,
 										  void *arg);
 
-extern bool process_equivalence(PlannerInfo *root, RestrictInfo *restrictinfo,
+extern bool process_equivalence(PlannerInfo *root,
+					RestrictInfo **p_restrictinfo,
 					bool below_outer_join);
 extern Expr *canonicalize_ec_expression(Expr *expr,
 						   Oid req_type, Oid req_collation);