[DAGCombiner] Don't fold zext_vector_inreg/sext_vector_inreg(undef) to undef. Fold to 0.

zext_vector_inreg needs to produces 0s in the extended bits and
sext_vector_inreg needs to produce upper bits that are all the
same. So we should fold them to a 0 vector instead of undef.

Fixes PR46585.