[GlobalOpt] Sort members of llvm.used deterministically

Patch by Jake VanAdrighem!

Summary:
Fix the way we sort the llvm.used and llvm.compiler.used members.

This bug seems to have been introduced in rL183756 through a set of improper casts to GlobalValue*. In subsequent patches this problem was missed and transformed into a getName call on a ConstantExpr.

Reviewers: silvas

Subscribers: silvas, llvm-commits

Differential Revision: http://reviews.llvm.org/D12851

llvm-svn: 248728