[FIX] Use unreachable to indicate dead code and repair dominance

  When we bail out early we make the partially build new code path
  practically dead, though it was not unreachable. To remove dominance
  problems we now make it not only dead but also prevent the control
  flow to join with the original code path, thus allow to use original
  values after the SCoP without any PHI nodes.

This fixes bug 25447.

llvm-svn: 252420