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

[via llvm-commits]

valadaptive wrote:

I've updated the PR description. I also noticed that for most other targets (except AMDGPU and x86), the `instCombineIntrinsic` hook is *not* a separate file, and is instead part of `[target]TargetTransformInfo.cpp`. Should I just move the new hook into `WebAssemblyTargetTransformInfo.cpp`?

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