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

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valadaptive wrote:

> InstCombine guide also asks for alive2 proof, I believe.

Is there a good guide for writing alive2 proofs? I'm not sure how to write a "for all arbitrary constants" constraint.

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