Support SCoPs with multiple exit edges

Regions that have multiple exit edges are very common. A simple if condition
yields e.g. such a region:

        if
      /   \
  then     else
      \   /
      after

Region: if -> after

This regions contains the bbs 'if', 'then', 'else', but not 'after'. It has
two exit edges 'then' -> 'after' and 'else' -> 'after'.

Previously we scheduled the RegionSimplify pass to translate such regions into
simple regions. With this patch, we now support them natively.

Contributed-by: Star Tan <tanmx_star@yeah.net>
llvm-svn: 179159