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

[Luke Lau via llvm-commits]

lukel97 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.

DAGCombiner has plenty of combines on shuffles so I don't think you'd miss much doing it there. But I'm just noticing now that this isn't actually a new pass, it's just implementing a TTI hook. Can you update the PR description to reflect that? I'm aware that the original comment in X86InstCombineIntrinsic.cpp says its a pass but that probably needs updated.

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