[FIX] Ensure SAI objects for exit PHIs

  If all exiting blocks of a SCoP are error blocks and therefor not
  represented we will not generate accesses and consequently no SAI
  objects for exit PHIs. However, they are needed in the code generation
  to generate the merge PHIs between the original and optimized region.
  With this patch we enusre that the SAI objects for exit PHIs exist
  even if all exiting blocks turn out to be eror blocks.

  This fixes the crash reported in PR27207.

llvm-svn: 265393