Tweak BRIN minmax operator class

In the union support proc, we were not checking the hasnulls flag of
value A early enough, so it could be skipped if the "allnulls" flag in
value B is set.  Also, a check on the allnulls flag of value "B" was
redundant, so remove it.

Also change inet_minmax_ops to not be the default opclass for type inet,
as a future inclusion operator class would be more useful and it's
pretty difficult to change default opclass for a datatype later on.
(There is no catversion bump for this catalog change; this shouldn't be
a problem.)

Extracted from a larger patch to add an "inclusion" operator class.

Author: Emre Hasegeli

src/test/regress/expected/brin.out

@@ -65,7 +65,7 @@ CREATE INDEX brinidx ON brintest USING brin (
 	float4col,
 	float8col,
 	macaddrcol,
-	inetcol,
+	inetcol inet_minmax_ops,
 	bpcharcol,
 	datecol,
 	timecol,

src/test/regress/sql/brin.sql

@@ -68,7 +68,7 @@ CREATE INDEX brinidx ON brintest USING brin (
 	float4col,
 	float8col,
 	macaddrcol,
-	inetcol,
+	inetcol inet_minmax_ops,
 	bpcharcol,
 	datecol,
 	timecol,