[llvm] [SelectionDAG] Fold VECREDUCE_ADD of a constant BUILD_VECTOR (PR #207560)

[Ankit Kumar Tiwari via llvm-commits]

ankit-cybertron wrote:

> > Folding is skipped if any element is undef
> 
> If an element is poison we can fold to poison though. Do we do that in this PR?

for now poison and undef both should return without folding. Poison could fold straight to a poison result instead. Not sure whether to add it here or as a follow-up

https://github.com/llvm/llvm-project/pull/207560