[PHIElimination] Update the regression test for PR16508

Summary:
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
  test/CodeGen/PowerPC/2013-07-01-PHIElimBug.mir
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.

Reviewers: MatzeB, hfinkel

Reviewed By: MatzeB

Subscribers: nemanjai, jsji, llvm-commits

Differential Revision: https://reviews.llvm.org/D52553

llvm-svn: 343416