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

[Petr Penzin via llvm-commits]

https://github.com/ppenzin commented:

InstCombine guide also asks for alive2 proof, I believe. @topperc, you know more about instcombine than I do, is this right structure for a new instcombine pass?

Not sure what our position on Claude, though in this case it is just for the test.


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