RegionInfo: add new test case

This test case has been reduced from test/Analysis/RegionInfo/mix_1.ll and
provides us with a minimal example of a test case which caused problems while
working on an improved version of the RegionInfo analysis. We upstream this
test case, as it certainly can be helpful in future debugging and optimization
tests.

Test case reduced by Pratik Bhatu <cs12b1010@iith.ac.in>

llvm-svn: 290974