Improve BRIN infra, minmax opclass and regression test

The minmax opclass was using the wrong support functions when
cross-datatypes queries were run.  Instead of trying to fix the
pg_amproc definitions (which apparently is not possible), use the
already correct pg_amop entries instead.  This requires jumping through
more hoops (read: extra syscache lookups) to obtain the underlying
functions to execute, but it is necessary for correctness.

Author: Emre Hasegeli, tweaked by Álvaro
Review: Andreas Karlsson

Also change BrinOpcInfo to record each stored type's typecache entry
instead of just the OID.  Turns out that the full type cache is
necessary in brin_deform_tuple: the original code used the indexed
type's byval and typlen properties to extract the stored tuple, which is
correct in Minmax; but in other implementations that want to store
something different, that's wrong.  The realization that this is a bug
comes from Emre also, but I did not use his patch.

I also adopted Emre's regression test code (with smallish changes),
which is more complete.

src/backend/access/brin/brin_tuple.c

@@ -68,7 +68,7 @@ brtuple_disk_tupdesc(BrinDesc *brdesc)
 		{
 			for (j = 0; j < brdesc->bd_info[i]->oi_nstored; j++)
 				TupleDescInitEntry(tupdesc, attno++, NULL,
-								   brdesc->bd_info[i]->oi_typids[j],
+								   brdesc->bd_info[i]->oi_typcache[j]->type_id,
 								   -1, 0);
 		}
 
@@ -444,8 +444,8 @@ brin_deform_tuple(BrinDesc *brdesc, BrinTuple *tuple)
 		for (i = 0; i < brdesc->bd_info[keyno]->oi_nstored; i++)
 			dtup->bt_columns[keyno].bv_values[i] =
 				datumCopy(values[valueno++],
-						  brdesc->bd_tupdesc->attrs[keyno]->attbyval,
-						  brdesc->bd_tupdesc->attrs[keyno]->attlen);
+						  brdesc->bd_info[keyno]->oi_typcache[i]->typbyval,
+						  brdesc->bd_info[keyno]->oi_typcache[i]->typlen);
 
 		dtup->bt_columns[keyno].bv_hasnulls = hasnulls[keyno];
 		dtup->bt_columns[keyno].bv_allnulls = false;