Tighten the PHI modeling test cases

Tighten the PHI modeling test cases

While looking through the test cases I realized we did not have a CHECK line
for a duplicate memory access which we may want to eliminate later. To ensure
we do not have (or later introduce) unnecessary memory accesses, we now tighten
the test cases to look for such a pattern (and add the CHECK: line that shows
the redundant memory access).

llvm-svn: 238227