AMDGPU/SI: Use flat for global load/store when targeting HSA

Summary:
  For some reason doing executing an MUBUF instruction with the addr64
  bit set and a zero base pointer in the resource descriptor causes
  the memory operation to be dropped when the shader is executed using
  the HSA runtime.

  This kind of MUBUF instruction is commonly used when the pointer is
  stored in VGPRs.  The base pointer field in the resource descriptor
  is set to zero and and the pointer is stored in the vaddr field.

  This patch resolves the issue by only using flat instructions for
  global memory operations when targeting HSA. This is an overly
  conservative fix as all other configurations of MUBUF instructions
  appear to work.

  NOTE: re-commit by fixing a failure in Codegen/AMDGPU/llvm.dbg.value.ll

Reviewers: tstellarAMD

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D15543

llvm-svn: 256282