Fix PR1938 by forcing the code that uses an undefined value to branch one

Fix PR1938 by forcing the code that uses an undefined value to branch one
way or the other.  Rewriting the code itself prevents subsequent analysis
passes from making contradictory conclusions about the code that could
cause an infeasible path to be made feasible.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46427 91177308-0d34-0410-b5e6-96231b3b80d8