Make the SCoP generation resistent wrt. error blocks

  When error blocks are not terminated by an unreachable they have successors
  that might only be reachable via error blocks. Additionally, branches in
  error blocks are not checked during SCoP detection, thus we might not be able
  to handle them. With this patch we do not try to model error block exit
  conditions. Anything that is only reachable via error blocks is ignored too,
  as it will not be executed in the optimized version of the SCoP anyway.

llvm-svn: 249099