[WebAssembly] Support for atomic fences

Summary:
This adds support for translation of LLVM IR fence instruction. We
convert a singlethread fence to a pseudo compiler barrier which becomes
0 instructions in final binary, and a thread fence to an idempotent
atomicrmw instruction to a memory address.

Reviewers: dschuff, jfb, sunfish, tlively

Subscribers: sbc100, jgravelle-google, llvm-commits

Differential Revision: https://reviews.llvm.org/D50277

llvm-svn: 361884