[llvm] [TTI][WebAssembly] Pairwise reduction expansion (PR #93948)

[Sam Parker via llvm-commits]

sparker-arm wrote:

> Is there a runtime that is expected to consume this already and how are you planning to disseminate this knowledge?

I'm currently working on a patch for AArch64 in V8, I don't _think_ there's been any similar work for other targets there. I guess there could be a risk of breaking some matching for cranelift's `iadd_pairwise` operation, but I haven't looked into it... I will ping those folks.

> Would it be a better approach to introduce a standard way of doing this?

There's a wasm spec post-SIMD-MVP [request](https://github.com/WebAssembly/simd/pull/375) for FP pairwise adds, which is what I would like.

https://github.com/llvm/llvm-project/pull/93948