Fix proposal for unsoundness issue #13 + test case

This patch addresses the unsoundness issue #13. The idea is to first collect the UB constraints and then add them together with the Phi node predicates. If no Phi nodes exist, add the UB constraints directly as instruction conditions. The recursive parsing at each Phi node can be improved, but we should discuss the proposed solution first.
There is nothing wrong with that optimization. You can find an explanation of the basic block and phi node semantics at:

https://github.com/google/souper/blob/master/docs/InstRef.rst#basic-blocks-and-phi-nodes

Is there anything there that does not make sense? I'm keen to make the docs clearer if it would help.