Implement the isSafeToDiscardIfUnused predicate and use it in globalopt and

Implement the isSafeToDiscardIfUnused predicate and use it in globalopt and
globaldce. Globaldce was already removing linkonce globals, but globalopt was
not.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158476 91177308-0d34-0410-b5e6-96231b3b80d8