[AtomicExpand] Allow libcall expansion for non-zero address spaces (try 2)

Restore a reverted commit, with the silly mistake fixed.  Sorry for the previous breakage.  

Be consistent about how we treat atomics in non-zero address spaces.  If we get to the backend, we tend to lower them as if in address space 0.  Do the same if we need to insert a libcall instead.

Differential Revision: https://reviews.llvm.org/D58760

llvm-svn: 355540