[WebAssembly] Improve readability of SIMD instructions (NFC)

Summary:
- Categorize instructions into the categories as in the SIMD spec
- Move SIMD-related definition to WebAssemblyInstrSIMD.td
- Put definition and use of patterns together
- Add newlines here and there

Reviewers: tlively

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

Differential Revision: https://reviews.llvm.org/D53045

llvm-svn: 344086