[tests] More testing for PHI-nodes in non-affine regions

We harden one test case by ensuring no additional stores may possibly be
introduced between the stores we check for and the basic block terminator
statements.

We also add a test case for the situation where a value that is passed from
a non-affine region to a PHI node does not dominate the exit of the non-affine
region. This case has come up in patch reviews, so we make sure it is properly
handled today and in the future.

llvm-svn: 250217