[CodeGen] avoid running the entire optimizer pipeline in clang test file; NFC

[CodeGen] avoid running the entire optimizer pipeline in clang test file; NFC

I'm making the CHECK lines vague enough that they pass at -O0.
If that is too vague (we really want to check the data flow
to verify that the variables are not mismatched, etc), then
we can adjust those lines again to more closely match the output
at -O0 rather than -O1.

This change is based on the post-commit comments for:
https://github.com/llvm/llvm-project/commit/83f4372f3a708ceaa800feff8b1bd92ae2c3be5f
http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20200224/307888.html