[aarch64][globalisel] Define G_ATOMIC_CMPXCHG and G_ATOMICRMW_* and make them legal

[aarch64][globalisel] Define G_ATOMIC_CMPXCHG and G_ATOMICRMW_* and make them legal

The IRTranslator cannot generate these instructions at the moment so there's no
issue with not having implemented ISel for them yet. D40092 will add
G_ATOMIC_CMPXCHG_WITH_SUCCESS and G_ATOMICRMW_* to the IRTranslator and a
further patch will add support for lowering G_ATOMIC_CMPXCHG_WITH_SUCCESS into
G_ATOMIC_CMPXCHG with an external success check via the `Lower` action.

The separation of G_ATOMIC_CMPXCHG_WITH_SUCCESS and G_ATOMIC_CMPXCHG is
to import SelectionDAG rules while still supporting targets that prefer to
custom lower the original LLVM-IR-like operation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319216 91177308-0d34-0410-b5e6-96231b3b80d8