GlobalOpt: Don't optimize thread_local for initializers

Folding a reference to a thread_local variable into another global
variable's initializer is very problematic, there is no relocation that
exists to represent such an access.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211762 91177308-0d34-0410-b5e6-96231b3b80d8