As mentioned in D93793, there are quite a few places where unary IRBuilder::CreateShuffleVector(X, Mask) can be used
instead of IRBuilder::CreateShuffleVector(X, Undef, Mask).
Let's update them.
Actually, it would have been more natural if the patches were made in this order:
(1) let them use unary CreateShuffleVector first
(2) update IRBuilder::CreateShuffleVector to use poison as a placeholder value (D93793)
The order is swapped, but in terms of correctness it is still fine.
ShadowTy is now an unused variable. Delete to avoid a compile warning.