[llvm] [DFAJumpThreading] Don't insert existing edge to DomTree while unfolding (PR #163296)

[Usman Nadeem via llvm-commits]

UsmanNadeem wrote:

> We may also need to delete the edge `StartBlock -> EndBlock` and insert the edge `StartBlock -> EndBlock` here:
> 
> https://github.com/llvm/llvm-project/blob/489a921796fe8d33de0f055ca6084e8f54cb1d84/llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp#L308-L310

We do this a few lines down:
```
    {{DominatorTree::Delete, StartBlock, EndBlock},
     {DominatorTree::Insert, StartBlock, NewBlockT}}
```

https://github.com/llvm/llvm-project/pull/163296