[CodeGen] Simplify conditional. NFC

Rafael pointed out that `hasInternalLinkage() || hasPrivateLinkage()` is
equivalent to `hasLocalLinkage()` in post-commit review.

I'm intentionally not updating the comment, partly because I like it
being explicit, and partly because "global symbols with local linkage"
sounds like an oxymoron.

llvm-svn: 323688