Fix const merging when an alias of a const is llvm.used.

We used to disable constant merging not only if a constant is llvm.used, but
also if an alias of a constant is llvm.used. This change fixes that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181175 91177308-0d34-0410-b5e6-96231b3b80d8