[Fix] Rewire the Region after a unconditional entry edge is created

  We use SplitEdge to split a conditional entry edge of the SCoP region.
  However, SplitEdge can cause two different situations (depending on
  whether or not the edge is critical). This patch tests
  which one is present and deals with the former unhandled one.

  It also refactors and unifies the case we have to change the basic
  blocks of the SCoP to new ones (see replaceScopAndRegionEntry).

llvm-svn: 217802