Use get_greatest_modulus() in partition_bounds_equal()

Hi all,

partition_bounds_equal() accesses the last datum from the given
partition bounds directly to compare their greatest moduli. Rest of
the code uses get_greatest_modulus() to get the greatest modulus from
a partition bound. partition_bounds_equal() should also do the same
for the sake of consistency and to make sure the in future the code
remains sane if we change the way we store greatest modulus in
PartitionBoundInfoData in future.

-- 
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company