AMDGPU/SI: Remove address space query functions from AMDGPUDAGToDAGISel

Summary:
These have been replaced with TableGen code (except for isConstantLoad,
which is still used for R600).  The queries were broken for cases
where MemOperand was a PseudoSourceValue.

Reviewers: arsenm

Subscribers: arsenm, kzhuravl, llvm-commits

Differential Revision: http://reviews.llvm.org/D21684

llvm-svn: 274561