Invalidate scop on encountering a complex control flow

We bail out if current scop has a complex control flow as this could lead to
building of large domain conditions. This is to reduce compile time.  This
addresses r26382.

Contributed-by: Chris Jenneisch <chrisj@codeaurora.org>

Differential Revision: http://reviews.llvm.org/D18362

llvm-svn: 264105