Clarify that llvm.used can contain aliases.

Also add a check for llvm.used in the verifier and simplify clients now that
they can assume they have a ConstantArray.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180019 91177308-0d34-0410-b5e6-96231b3b80d8