Reset, not recreate, execGrouping.c style hashtables.

Reset, not recreate, execGrouping.c style hashtables.

This uses the facility added in the preceding commit to fix
performance issues caused by rebuilding the hashtable (with its
comparator expression being the most expensive bit), after every
reset. That's especially important when the comparator is JIT
compiled.

Bug: #15592 #15486
Reported-By: Jakub Janeček, Dmitry Marakasov
Author: Andres Freund
Discussion:
    https://postgr.es/m/15486-05850f065da42931@postgresql.org
    https://postgr.es/m/20190114180423.ywhdg2iagzvh43we@alap3.anarazel.de
Backpatch: 11, where I broke this in bf6c614a2f2c5