Don't replace an alias in llvm.used with its target.

When we replace an internal alias with its target, be careful not to
replace the entry in llvm.used (and llvm.compiler_used).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181524 91177308-0d34-0410-b5e6-96231b3b80d8