[PowerPC] add memcmp test with nobuiltin attr; NFC

In SDAG, we don't expand libcalls with a nobuiltin attribute.
It's not clear if that's correct from the existing code comment:
"Don't do the check if marked as nobuiltin for some reason."

...adding a test here either way to show that there is currently
a different behavior implemented in the CGP-based expansion.

llvm-svn: 304991