[AMDGPU] Add an experimental buffer fat pointer address space.

[AMDGPU] Add an experimental buffer fat pointer address space.

Add an experimental buffer fat pointer address space that is currently
unhandled in the backend. This commit reserves address space 7 as a
non-integral pointer repsenting the 160-bit fat pointer (128-bit buffer
descriptor + 32-bit offset) that is heavily used in graphics workloads
using the AMDGPU backend.

Differential Revision: https://reviews.llvm.org/D58957

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356373 91177308-0d34-0410-b5e6-96231b3b80d8