AMDGPU/SI: Don't crash on anonymous GlobalValues

Summary:
We need to call AsmPrinter::getNameWithPrefix() in order to handle
anonymous GlobalValues (e.g. @0, @1).

Reviewers: arsenm, b-sumner

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl, tony-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D24865

llvm-svn: 282420