Fix assertion due to loop overlap with nonaffine region.

Reject and report regions that contains loops overlapping nonaffine region.
This situation typically happens in the presence of inifinite loops.

This addresses bug llvm.org/PR28071.

Differential Revision: http://reviews.llvm.org/D21312



Contributed-by: Huihui Zhang <huihuiz@codeaurora.org>
llvm-svn: 273905