the hot path contained a call to

    v `elemUnVarSet` (neighbors g v)

and creating the set of neighbors just to check if `v` is inside
accounted for half the allocations of the test case of #15164.

By introducing a non-allocating function `hasLoopAt` for this we shave
off half the allocations. This brings the total cost of Call Arity down
to 20% of time and 23% of allocations, according to a profiled run. Not
amazing, but still much better.

Differential Revision: https://phabricator.haskell.org/D4718