[llvm] [WebAssembly] v16i8 mul support (PR #150209)

[Sam Parker via llvm-commits]

sparker-arm wrote:

In writing code gen tests for #146864 , I realised that we don't have a v16i8 mul instruction and the code gen looked bad. So, I'd like to get this fixed first.

https://github.com/llvm/llvm-project/pull/150209