When PR16508 was solved (in rL185363) a regression test was
added as test/CodeGen/PowerPC/2013-07-01-PHIElimBug.ll.
I discovered that the test case no longer reproduced the
scenario from PR16508. This problem could have been amended
by adding an extra RUN line with "-O1" (or possibly "-O0"),
but instead I added a mir-reproducer
to get a reproducer that is less sensitive to changes in
earlier passes (including O-level).
While being at it I also corrected a code comment in
PHIElimination::EliminatePHINodes that has been incorrect
since the related bugfix from rL185363.