Add SIMD128 support to the wasm backend
Most WebAssembly runtimes as of today supports 128bits fixed-width SIMD, and it's also supported by LLVM. So it's possible for us to support it in the wasm backend, a good portion of GHC SIMD primops can be lowered to wasm SIMD128 opcodes directly.
I don't see any particular difficulty in this, but it does take some time to implement. And it's a rather low priority feature request, should be postponed given: