[llvm] [WebAssembly] Fold constant `i8x16.swizzle` and `i8x16.relaxed.swizzle` to `shufflevector` (PR #169110)

[via llvm-commits]

valadaptive wrote:

> Is it possible to do this as a DAG combine and avoid adding a new pass

My understanding is that DAG combine passes run much later in the pipeline, so we'd be losing out on a lot of optimizations if this was a DAG combine.

https://github.com/llvm/llvm-project/pull/169110