Split invocations in CodeGen/X86/cpus.ll among multiple tests. (NFC)

On GreenDragon `CodeGen/X86/cpus.ll` is timing out on the bot with Asan
and UBSan enabled. With the same configuration on my machine, the test
passes but takes more than 3 minutes to do so. I could increase the
timeout, but I believe it makes more sense to split up the test because
it allows for more parallelism.

Differential revision: https://reviews.llvm.org/D52603

llvm-svn: 343313