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.
I don't think that would be quite right. Consider the following:

    %0 = block
    %1:i32 = var
    %2:i32 = var
    %3 = addnsw %1, %2
    %4 = phi %0, %3, 0
    %5 = block
    %6 = phi %5, %4, 0
    cand %6 0

In this case, the UB would be conditional on the predicates for both ```%0``` and ```%5```.