Create parallel code in a separate block

  This commit basically reverts r246427 but still solves the issue
  tackled by that commit. Instead of emitting initialization code in the
  beginning of the start block we now generate parallel code in its own
  block and thereby guarantee separation. This is necessary as we cannot
  generate code for hoisted loads prior to the start block but it still
  needs to be placed prior to everything else.

llvm-svn: 248674