[WebAssembly][NFC] Reorder SIMD section

Summary:
Reorders the sections in the SIMD tablegen file to roughly match the
new opcode ordering. Depends on D54126.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D54134

llvm-svn: 346464