Adjust regression test to avoid platform-dependent failure.

We have a test that verifies that max(anyarray) will cope if the array
column elements aren't all the same array type.  However, it's now possible
for that to produce a collation-related error message instead of the
expected one, if the first two column elements happen to be of the same
type and it's one that expects to be given collation info.  Tweak the test
to ensure this doesn't happen.  Per buildfarm member pika.

src/test/regress/expected/polymorphism.out

@@ -623,7 +623,8 @@ where histogram_bounds is not null;
 (1 row)
 
 -- such functions must protect themselves if varying element type isn't OK
-select max(histogram_bounds) from pg_stats;
+-- (WHERE clause here is to avoid possibly getting a collation error instead)
+select max(histogram_bounds) from pg_stats where tablename = 'pg_am';
 ERROR:  cannot compare arrays of different element types
 -- test variadic polymorphic functions
 create function myleast(variadic anyarray) returns anyelement as $$

src/test/regress/sql/polymorphism.sql

@@ -433,7 +433,8 @@ select distinct array_ndims(histogram_bounds) from pg_stats
 where histogram_bounds is not null;
 
 -- such functions must protect themselves if varying element type isn't OK
-select max(histogram_bounds) from pg_stats;
+-- (WHERE clause here is to avoid possibly getting a collation error instead)
+select max(histogram_bounds) from pg_stats where tablename = 'pg_am';
 
 -- test variadic polymorphic functions