[FIX] Create error-restrictions late

  Before this patch we generated error-restrictions only for
  error-blocks, thus blocks (or regions) containing a not represented
  function call. However, the same reasoning is needed if the invalid
  domain of a statement subsumes its actual domain. To this end we move
  the generation of error-restrictions after the propagation of the
  invalid domains. Consequently, error-statements are now defined more
  general as statements that are assumed to be not executed.
  Additionally, we do not record an empty domain for such statements but
  a nullptr instead. This allows to distinguish between error-statements
  and dead-statements.

llvm-svn: 269053