[WebAssembly] AsmParser: better atomic inst detection

Summary:
Previously missed atomic.notify.

Fixes https://bugs.llvm.org/show_bug.cgi?id=40728

Reviewers: aheejin

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63747

llvm-svn: 364576