[DTU] fix dominator tree update eliding reachable nodes (#177683)
The initial CFG looks like this:
After inlining, it looks like this:
It should be sufficient to add and remove the edges shown in the test, i.e.:
- add: `bb3->bb1.i` and `bb3->bb2.i`
- remove: `bb3->bb4`, `bb3->bb5` and `bb5->bb8`
New nodes, like `bb5.body`, get discovered when adding bb3->bb2.i. See the "StepByStep" variant of the test). Without the fix in this patch, however, `bb5.body` gets elided when the deleted edges get taken into account, and `DT` is left invalid.
llvm-project
ded64d24 - [DTU] fix dominator tree update eliding reachable nodes (#177683)
