GlobalOpt: Don't optimize dllimport for initializers

Referencing a dllimport variable requires actually instructions, not
just a relocation.  This fixes PR19955.

Differential Revision: http://reviews.llvm.org/D4249

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211571 91177308-0d34-0410-b5e6-96231b3b80d8