[WebAssembly] Fix load/store name detection for atomic instructions
Summary:
Fixed a bug in the routine in AsmParser that determines whether the
current instruction is a load or a store. Atomic instructions' prefixes
are not `atomic_` but `atomic.`, and all atomic instructions are also
memory instructions. Also fixed the printing format of atomic
instructions to match other memory instructions and added encoding tests
for atomic instructions.
Reviewers: aardappel, tlively
Subscribers: dschuff, sbc100, jgravelle-google, sunfish, jfb, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D58337
llvm-svn: 354419