Add more benchmarks for literal strings.

Summary:
Comparing against the empty string should generate much better code that
what it does today.
We can also generate better code when comparing against literals that
are larger than the SSO space.

Reviewers: EricWF

Subscribers: christof, jdoerfert, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D59781

llvm-svn: 357614