[llvm] [SimplifyCFG] Enable MergeBlockIntoPredecessor with two successors (PR #143766)

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HighW4y2H3ll wrote:

Thanks for checking!! Sorry that I missed a commit earlier which actually uses the PredecessorWithTwoSuccessors. The `speculativelyExecuteBB` transformation seems to be more restrictive to the instructions: e.g. only allowing conditional load/store, only speculate merge basic block with one additional instruction etc. I'm checking the FlattenCFG pass right now. Maybe that's actually the right place for such transformation...

https://github.com/llvm/llvm-project/pull/143766