InvariantsCPA: Fixed two bugs in the computation of the value of a boolean flag; a) the expression was inverse to the correct expression, b) the statement was in a location where the expression would always evaluate to the same result.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@12935 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c