[x86] NFC: Add tests for idiomatic usage patterns of SSE4.2 string

comparison instructions (pcmp[ei]stri*).

These will help show improvements from fixes to PR37246.

I've not really covered the mask forms of this intrinsic as I don't have
as good of an intuition about the likely usage patterns there. Happy for
someone to extend this with tests covering the mask form.

llvm-svn: 330895