AMDGPU: Make 0 the private nullptr value

We can't actually pretend that 0 is valid for address space 0.
r295877 added a workaround to stop allocating user objects
there, so we can use 0 as the invalid pointer.

Some of the tests seemed to be using private as the non-0 null
test address space, so add copies using local to make sure
this is still stressed.

llvm-svn: 297659