[WebAssembly] Massive instruction renaming

Summary:
An automated renaming of all the instructions listed at
https://github.com/WebAssembly/spec/issues/884#issuecomment-426433329
as well as some similarly-named identifiers.

Reviewers: aheejin, dschuff, aardappel

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

Differential Revision: https://reviews.llvm.org/D56338

llvm-svn: 350609