[AMDGPU] Fix pointer info for lowering load/store for r600 for amdgiz environment

r600 uses dummy pointer info for lowering load/store. Since dummy pointer info
assumes address space 0, this causes isel failure when temporary load/store SDNodes
are generated for amdgiz environment.

Since the offest is not constant, FixedStack pseudo source value cannot be used
to create the pointer info. This patch creates pointer info using llvm undef value.
At least this provides correct address space so that isel can be done correctly.

Differential Revision: https://reviews.llvm.org/D39698

llvm-svn: 317862