[llvm] [SimplifyCFG] Introduce a heuristic code sinker to fold phi expressions (PR #128171)

[via llvm-commits]

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@@ -2423,6 +2410,27 @@ static void sinkLastInstruction(ArrayRef<BasicBlock*> Blocks) {
   }
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sharkautarch wrote:

I recently noticed that llvm/clang built w/ this draft PR merged in is now sometimes crashing
I verified that `IncomingBBs.size() == Insts.size()`, and that all I in `Insts` had the same number of operands as I0 by adding checks into `sinkCandidatesImpl()`. 
Note that I didn't build llvm w/ asserts on, I just manually added `if (cond) { fprintf(stderr, "cond\n"); abort(); }`
I ended up figuring out that somehow `I->getParent()` is sometimes returning `nullptr` here...

https://github.com/llvm/llvm-project/pull/128171