Add single entry / single exit accessors.

Add methods for accessing the (single) entry / exit edge of a region. If no such
edge exists, null is returned.  Both accessors return the start block of the
corresponding edge. The edge can finally be formed by utilizing
Region::getEntry() or Region::getExit();

Contributed by: Andreas Simbuerger <simbuerg@fim.uni-passau.de>

llvm-svn: 123410