AMDGPU/SI: Make sure not to fold offsets into local address space globals

Summary:
Offset folding only works if you are emitting relocations, and we don't
emit relocations for local address space globals.

Reviewers: arsenm, nhaustov

Subscribers: arsenm, llvm-commits, kzhuravl

Differential Revision: http://reviews.llvm.org/D21647

llvm-svn: 273765