Codegen: Admissible inames finder broken for items not in any subkernel

The intent of this loop is to find the subkernel containing the schedule item:

https://gitlab.tiker.net/inducer/loopy/blob/master/loopy/codegen/bounds.py#L72

When the schedule item isn't in a subkernel, this just finfs the most recent subkernel (modulo wraparound). This could lead to overapproximating the set of admissible inames.

I fixed this in !107, but seeing as that MR has other problems that prevent it from being merged it might be worth fixing this issue separately.