Injection points for hash aggregation.
Requires adding a guard against shift-by-32. Previously, that was
impossible because the number of partitions was always greater than 1,
but a new injection point can force the number of partitions to 1.
Discussion: https://postgr.es/m/
ff4e59305e5d689e03cd256a736348d3e7958f8f.camel@j-davis.com