[WebAssembly] Add missing _S opcodes of atomic stores to InstPrinter

Summary: This was missing in D48839 (rL336145).

Reviewers: aardappel

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

Differential Revision: https://reviews.llvm.org/D48992

llvm-svn: 336390