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

[Jasmine Tang via llvm-commits]

badumbatish wrote:

thanks for the PR, looks really good to me, I think the code formatter is complaining about undef usages in one of the test, we can change it to poison for those.

Let's wait for another reviewer and for all the CI to pass

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