Fixed a bug in the single loop transformation introduced by a recent change: The initial subgraph actually must be syntactically reachable from the loop head, otherwise there might be multiple loop entry edges, which is not supported by the LoopstackCPA.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@12975 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c