[llvm] [SimplifyCFG] Reland transform for redirecting phis between unmergeable BB and SuccBB (PR #68473)

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hiraditya wrote:

This patch still has a couple of bugs as exposed in #102351
1. CanPropagatePredecessorsForPHIs can't handle indirect branch
2. the set intersection algorithm (to find common pred) has a bug.

https://github.com/llvm/llvm-project/pull/68473