This will be used by pending patches to improve partition pruning.

Amit Langote and Kyotaro Horiguchi, per a suggestion from David
Rowley.  Review and testing of the larger patch set of which this is a
part by Ashutosh Bapat, David Rowley, Dilip Kumar, Jesper Pedersen,
Rajkumar Raghuwanshi, Beena Emerson, Amul Sul, and Kyotaro Horiguchi.

Discussion: http://postgr.es/m/098b9c71-1915-1a2a-8d52-1a7a50ce79e8@lab.ntt.co.jp