[WebAssembly] Renumber SIMD ops

Summary:
This change leaves holes in the opcode space where missing
instructions could logically be added later if they were found to be
useful.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D52282

llvm-svn: 342610