[SimplifyCFG] simplifySingleResume(): FoldReturnIntoUncondBranch() already...

[SimplifyCFG] simplifySingleResume(): FoldReturnIntoUncondBranch() already knows how to preserve DomTree

... so just ensure that we pass DomTreeUpdater it into it.

Apparently, there were no dedicated tests just for that functionality,
so i'm adding one here.