jit: Re-allow JIT compilation of execGrouping.c hashtable comparisons.

In the course of 5567d12c, 356687bd and 317ffdfe, I changed
BuildTupleHashTable[Ext]'s call to ExecBuildGroupingEqual to not pass
in the parent node, but NULL. Which in turn prevents the tuple
equality comparator from being JIT compiled.  While that fixes
bug #15486, it is not actually necessary after all of the above commits,
as we don't re-build the comparator when using the new
BuildTupleHashTableExt() interface (as the content of the hashtable
are reset, but the TupleHashTable itself is not).

Therefore re-allow jit compilation for callers that use
BuildTupleHashTableExt with a separate context for "metadata" and
content.

As in the previous commit, there's ongoing work to make this easier to
test to prevent such regressions in the future, but that
infrastructure is not going to be backpatchable.

The performance impact of not JIT compiling hashtable equality
comparators can be substantial e.g. for aggregation queries that
aggregate a lot of input rows to few output rows (when there are a lot
of output groups, there will be fewer comparisons).

Author: Andres Freund
Discussion: https://postgr.es/m/20190927072053.njf6prdl3vb7y7qb@alap3.anarazel.de
Backpatch: 11, just as 5567d12c