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

[Sam Parker via llvm-commits]

sparker-arm wrote:

> Would it be portable and fast if we specified that the "undef" lanes all end up containing zeros, for instance?

This is the only option that I have considered, really. It would then have the same semantics as what I'm proposing here and I would expect it is cheap enough on any architecture.

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