[globalisel][irtranslator] Add support for atomicrmw and (strong) cmpxchg

Summary:
This patch adds support for the atomicrmw instructions and the strong
cmpxchg instruction to the IRTranslator.

I've left out weak cmpxchg because LangRef.rst isn't entirely clear on what
difference it makes to the backend. As far as I can tell from the code, it
only matters to AtomicExpandPass which is run at the LLVM-IR level.

Reviewers: ab, t.p.northover, qcolombet, rovka, aditya_nandakumar, volkan, javed.absar

Reviewed By: qcolombet

Subscribers: kristof.beyls, javed.absar, igorb, llvm-commits

Differential Revision: https://reviews.llvm.org/D40092

llvm-svn: 336589