fix test to not check optimized IR; NFCI

This test broke with an LLVM instcombine patch (r297166).
I changed the RUN line to only run -mem2reg (to save time checking this large chunk of tests)
and updated the checks using the script attached to D17999:
https://reviews.llvm.org/D17999

The goal is to make this test immune to optimizer changes. If there's something in these
tests that was checking for an IR optimization, that should be tested in LLVM, not Clang.

llvm-svn: 297189