Revert "[LoopUnroll] Directly update DT instead of DTU."

This reverts commit d0907ce7.

Causes `opt -passes=loop-unroll-full` to crash on

```
define void @foo() {
bb:
  br label %bb1

bb1:                                              ; preds = %bb1, %bb1, %bb
  switch i1 true, label %bb1 [
    i1 true, label %bb2
    i1 false, label %bb1
  ]

bb2:                                              ; preds = %bb1
  ret void
}
```