[globalisel][tablegen] Add support for relative AtomicOrderings

No test yet because the relevant rules are blocked on the atomic_load,
and atomic_store nodes.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319475 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed