[WebAssembly] Add atomic.fence instruction

[WebAssembly] Add atomic.fence instruction

Summary:
This adds `atomic.fence` instruction:
https://github.com/WebAssembly/threads/blob/master/proposals/threads/Overview.md#fence-operator

And we now emit the new `atomic.fence` instruction for multithread
fences, rather than the prevous `atomic.rmw` hack.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, hiraditya, sunfish, jfb, tlively, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66794

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@370272 91177308-0d34-0410-b5e6-96231b3b80d8