[WebAssembly] Update relaxed SIMD opcodes and names

to reflect the latest state of the proposal:
https://github.com/WebAssembly/relaxed-simd/blob/main/proposals/relaxed-simd/Overview.md#binary-format.
Moves code around to match the instruction order from the proposal, but the only
functional changes are to the names and opcodes.

Reviewed By: aheejin

Differential Revision: https://reviews.llvm.org/D125726