It's called a Depth-First tree after all, not a
dept-first-but-feel-free-to-record-any-of-the-parents tree. In
particular, in order for the semi-dominator condition to hold,
the parent in the DFS tree needs to be the predecessor with the
highest DFS number. Here we were accidentally doing the opposite
casuing us to look at the wrong node in case the sdom and the idom
are not the same. To understand #31121, consider the following
CFG (minimized from the bug report to show the issue), as well as
the corresponding DFS numbering and sdom assignment

```
     CFG     DFS     sdom

      A       1      0
      | \     | \    |\
      B C     2 5    1 1
     /|/     /|/    /|/
    | D     | 3    | 1
     \|      \|     \|
      E       4      2
```

This bug caused us to record the parent of `E` as `B`, when it should
have been `D` (the relevant invariant here is that the parent in the DFS
tree is the predecessor with the highest DFS number).
As a result, when computing idoms from the sdoms, we
were incorrectly looking at `B`, seeing that the sdom matched the
ancestor in the DFS tree and thus concluding that `E`'s idom was `B`
rather than `A`. As a result, we neglected to insert a phi node in
`E`.

Fixes #31121