ScopInfo: Bail out in case of complex branch structures

Scops that contain many complex branches are likely to result in complex domain
conditions that consist of a large (> 100) number of conjucts.  Transforming
such domains is expensive and unlikely to result in efficient code.  To avoid
long compile times we detect this case and skip such scops. In the future we may
improve this by either using non-affine subregions to hide such complex
condition structures or by exploiting in certain cases properties (e.g.,
dominance) that allow us to construct the domains of a scop in a way that
results in a smaller number improving conjuncts.

Example of a code that results in complex iteration spaces:

      loop.header
     /    |    \ \
   A0    A2    A4 \
     \  /  \  /    \
      A1    A3      \
     /  \  /  \     |
   B0    B2    B4   |
     \  /  \  /     |
      B1    B3      ^
     /  \  /  \     |
   C0    C2    C4   |
     \  /  \  /    /
      C1    C3    /
       \   /     /
    loop backedge

llvm-svn: 256123