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

[Thomas Lively via llvm-commits]

tlively wrote:

Is the idea here that the Wasm engine can recognize the special shuffle pattern and generate different code for it? If so, is that code the engine is expected to generate correct with respect to the Wasm spec, or is this a method of providing unsafe optimization hints to the engine?

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