[DAG] Fold (vt trunc (extload (vt x))) -> (vt load x) (#75229)

We were only folding cases which remained extloads, but DAG.getExtLoad can also handle the cases which don't need to extend at all (we just can't do truncloads).

reduceLoadWidth can handle this for scalar loads, but not for vectors.

Noticed while triaging D152928