Allow PHI nodes in the region exit block

  While we do not need to model PHI nodes in the region exit (as it is not part
  of the SCoP), we need to prepare for the case that the exit block is split in
  code generation to create a single exiting block. If this will happen, hence
  if the region did not have a single exiting block before, we will model the
  operands of the PHI nodes as escaping scalars in the SCoP.

Differential Revision: http://reviews.llvm.org/D12051

llvm-svn: 247078