CodeGen: Fix pointer info in expandUnalignedLoad/Store

Currently expandUnalignedLoad/Store uses place holder pointer info for temporary memory operand
in stack, which does not have correct address space. This causes unaligned private double16 load/store to be
lowered to flat_load instead of buffer_load for amdgcn target.

This fixes failures of OpenCL conformance test basic/vload_private/vstore_private on target amdgcn---amdgizcl.

Differential Revision: https://reviews.llvm.org/D35361

llvm-svn: 314566