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

[Derek Schuff via llvm-commits]

dschuff wrote:

Can you say a little more about what the advantages of this are, i.e. what the VM does differently as a result? (And, which VMs have you tested this with?)

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