[NFCI][Local] TryToSimplifyUncondBranchFromEmptyBlock(): improve Dominator Tree updating

First, we don't need vector-ness for the predecessor lists.

Secondly, like elsewhere, do insertions before deletions.

Lastly, the check that we actually need to insert an edge,
that it doesn't exist already, is backwards. Instead of
looking at successors of every single 'PredOfBB',
just always look at predecessors of the 'Succ'.
The result is always the same, but we avoid *really* inefficient code.