Make MergeBlockIntoPredecessor conformant to the precondition of calling DTU.applyUpdates
It is mentioned in the document of DTU that "It is illegal to submit any update that has already been submitted, i.e., you are supposed not to insert an existent edge or delete a nonexistent edge." It is dangerous to violet this rule because DomTree and PostDomTree occasionally crash on this scenario.
This patch fixes MergeBlockIntoPredecessor, making it conformant to this precondition.
Reviewers: kuhar, brzycki, chandlerc
Reviewed By: brzycki
Subscribers: hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D58444