[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346464 91177308-0d34-0410-b5e6-96231b3b80d8

Author: tlively