[SimpleLoopUnswitch] Fix DT updates for trivial branch unswitching.
Summary:
Fixing 2 issues with the DT update in trivial branch switching, though I don't have a case where DT update fails.
1. After splitting ParentBB->UnswitchedBB edge, new edges become: ParentBB->LoopExitBB->UnswitchedBB, so remove ParentBB->LoopExitBB edge.
2. AFAIU, for multiple CFG changes, DT should be updated using batch updates, vs consecutive addEdge and removeEdge calls.
Reviewers: chandlerc, kuhar
Subscribers: sanjoy, jlebar, llvm-commits
Differential Revision: https://reviews.llvm.org/D49925
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@338180
91177308-0d34-0410-b5e6-
96231b3b80d8
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