limit check insertion via loop forests

[Matthew Fluet]

> B.t.w., there is one complication with Steve's represenation of loop
> forests, which is that all of the nodes that aren't in any loops are
> in the same notInLoop vector (at the top level) are effectively in the
> same loop.  Not a big deal, when setting up the equivalence classes, we
> just need to specially treat those in the top notInLoop vector.

Actually, this was a big deal with the original definition of L, because
every label that followed a non-loop label would be put into L.  But, as I
said, it's just special treatment of those in the top level.