Crash on on mcs_spinlock with pso

[margnus1]

This test case was the reproduction for #42, but after fixing it another issue cropped up. With several older LLVM versions, f.ex. 3.8, the test case now crashes with:

nidhugg: /home/magnus/.local/lib/llvm/3.8/include/llvm/ADT/ilist.h:264: llvm::ilist_iterator<NodeTy>& llvm::ilist_iterator<NodeTy>::operator--() [with NodeTy = llvm::Instruction]: Assertion `NodePtr && "--'d off the beginning of an ilist!"' failed.