[DAG] Don't convert undef to 0 when creating buildvector

When inserting undef into buildvectors created from shuffles of
buildvectors, we convert elements to the largest needed type. This had
the effect of converting undef into 0, which isn't needed as the
buildvector implicitly truncates and trunc(zext(undef)) == undef.

Differential Revision: https://reviews.llvm.org/D121002