ADT: Add some missing coverage for iplist::splice

These splices are interesting because they involve swapping two nodes in
the same list.  There are two ways to do this.  Assuming:

    A -> B -> [Sentinel]

You can either:
- splice B before A, with:        L.splice(A,       L, B) or
- splice A before Sentinel, with: L.splice(L.end(), L, A) to create:

    B -> A -> [Sentinel]

These two swapping-splices are somewhat interesting corner cases for
maintaining the list invariants.  The tests pass even with my new ilist
implementation, but I had some doubts about the latter when I was
looking at weird UB effects.  Since I can't find equivalent explicit
test coverage elsewhere it seems prudent to commit.

llvm-svn: 278887