79ae407c95
As noted in https://github.com/llvm/llvm-project/pull/141821#issuecomment-2917328924, whilst we currently constant fold intrinsics of fixed-length vectors via their scalar counterpart, we don't do the same for scalable vectors. This handles the scalable vector case when the operands are splats. One weird snag in ConstantVector::getSplat was that it produced a undef if passed in poison, so this also contains a fix by checking for PoisonValue before UndefValue.