[SimplifyCFG] Simplify identical predecessors (#173022) When >1 predecessors of BB are identical, try to merge them into ONE. --- Here is a simplified example (`sink` and `bb*`s share the same predecessor `entry`, hindering the existing uncond br folding to optimize such a case): ```diff - entry: - switch to %br1, %br2, %br3, %sink - bb1: - br label %sink - bb2: - br label %sink - bb3: - br label %sink - sink: - %ret = phi i8 [ 0, %bb1 ], [ 0, %bb2 ], [ 0, %bb3 ], [ -1, %entry ] + entry: + switch to %br1, %sink + bb1: + br label %sink + sink: + %ret = phi i8 [ 0, %bb1 ], [ -1, %entry ] ``` Actually, `simplifyDuplicateSwitchArms` did similar things in a very limited scope (only for switch arms), this patch generalizes its logic to handle any BB with >1 identical predecessors. --- This PR lands the [discussion](https://github.com/dtcxzyw/llvm-opt-benchmark/pull/3033#discussion_r2506973226), i.e., "merge identical predecessor bottom to up", and implements the suggestion of https://github.com/llvm/llvm-project/pull/114262#issuecomment-2448140669. - IR diff: https://github.com/dtcxzyw/llvm-opt-benchmark/pull/3537 - CompTime Impact: https://github.com/dtcxzyw/llvm-opt-benchmark/pull/3538
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