[WebAssembly] Renumber SIMD opcodes

Summary:
As described in https://github.com/WebAssembly/simd/pull/209. This is
the final reorganization of the SIMD opcode space before
standardization. It has been landed in concert with corresponding
changes in other projects in the WebAssembly SIMD ecosystem.

Reviewers: aheejin

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79224