[SystemZ] Eliminate unnecessary serialization operations

We currently emit a serialization operation (bcr 14, 0) before every
atomic load and after every atomic store.  This is overly conservative.
The SystemZ architecture actually does not require any serialization
for atomic loads, and a serialization after an atomic store only if
we need to enforce sequential consistency.  This is what other compilers
for the platform implement as well.

llvm-svn: 310093