[WebAssembly] Refactor atomic operation definitions (NFC)

Summary:
- Make `ATOMIC_I`, `ATOMIC_NRI`, `AtomicLoad`, `AtomicStore` classes and
  make other operations inherit from them
- Factor the common opcode prefix '0xfe' out from the opcodes into the
  common class
- Reorder instructions in the order of increasing opcodes

Reviewers: tlively

Subscribers: dschuff, sbc100, jgravelle-google, sunfish, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58338

llvm-svn: 354421