[CodeGen] update test file to not run the entire LLVM optimizer; NFC

Clang regression tests should not break when changes are made to
the LLVM optimizer. This file broke on the 1st attempt at D110170,
so I'm trying to prevent that on another try.

Similar to other files in this directory, we make a compromise and
run -mem2reg to reduce noise by about 1000 lines out of 5000+ CHECK lines.