[AMDGPU] Switch address space mapping by triple environment amdgiz

For target environment amdgiz and amdgizcl (giz means Generic Is Zero), AMDGPU will use new address space mapping where generic address space is 0 and private address space is 5. The data layout is also changed correspondingly.

Differential Revision: https://reviews.llvm.org/D31210

llvm-svn: 298767