Support SCoPs with multiple entry edges.

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

  if
 /   \
then  else
 \   /
for_region

This for_region contains two entry edges 'then' -> 'for_region' and 'else' -> 'for_region'.

Previously we scheduled the RegionSimplify pass to translate such regions into
simple regions. With this patch, we now support them natively when the region is
in -loop-simplify form, which means the entry block should not be a loop header.

Contributed by:  Star Tan <tanmx_star@yeah.net>

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@179586 91177308-0d34-0410-b5e6-96231b3b80d8