[WebAssembly] Make disassembler always emit most canonical name.

Summary:
There are a few instructions that all map to the same opcode, so
when disassembling, we have to pick one. That was just the first one
before (the except_ref variant in the case of "call"), now it is the
one marked as IsCanonical in tablegen, or failing that, the shortest
name (which is typically the "canonical" one).

Also introduced a canonical "end" instruction for this purpose.

Reviewers: dschuff, tlively

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57713

llvm-svn: 353131