[WebAssembly][NFC] Reorganize SIMD instructions

Summary:
Reorganize WebAssemblyInstrSIMD.td to put all of the instruction
definitions together, making it easier to see which instructions have
been implemented already. Depends on D51143.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D51113

llvm-svn: 340504