[llvm] [WebAssembly] Mask undef shuffle lanes (PR #149084)

[Sam Parker via llvm-commits]

sparker-arm wrote:

I've been trying to get some more performance results and I can't even reduce the AArch64 results I previously reported... I fear I may have used the toolchain from #150209 instead :'(

I've also noticed we'd need another patch in V8 to, hopefully, demonstrate the benefits. So, I'll leave this as a draft until everything is in order. Sorry for the noise!

https://github.com/llvm/llvm-project/pull/149084