[llvm] [Coroutines] Make CoroElide actually work for (trivial) C++ coroutines (PR #92969)

[via llvm-commits]

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@@ -253,6 +283,44 @@ void CoroIdElider::elideHeapAllocations(uint64_t FrameSize, Align FrameAlign) {
   removeTailCallAttribute(Frame, AA);
 }
 
+static BasicBlock *getNullCheckerPredecessor(Value *Handle,
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WenleiHe wrote:

> However, we need to think about more robust ways other than brute force pattern matching.

I actually that the way it's handle is okay. At least I don't see correctness issue IIUC. 

To formulate the problem/solution, we are essentially trying to mark paths safe/non-escape that are either 1) destroyed, or 2) null. null is okay obviously as nothing is escaping. The way you implement this is by hoisting the the safe/visited marking to the dominator of 1) and 2), not the most elegant, but does its job IMHO. 

https://github.com/llvm/llvm-project/pull/92969