Add testcase for SCEV explansion in non-affine subregions

When sharing the same map from old to new value, CodeGeneration would
reuse the same new value for each basic block. However, the SCEV
expander might emit code in a basic block that does not dominate a use
of the SCEV in another basic block. This test checks whether both such
blocks have their own expanded new values. 

llvm-svn: 250389