[llvm] [SimplifyCFG] Fix hoisting problem in SimplifyCFG (PR #78615)

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RouzbehPaktinat wrote:

> > > Causes a crash in stage2 clang: https://llvm-compile-time-tracker.com/show_error.php?commit=e36825ea6feefeef44cca7710d775fba287d3fb2
> > 
> > 
> > Hi @nikic It should be fixed now.
> 
> Could you add the corresponding test cases? I'd like to know what the problem is here. Is this it?
> 
> ```diff
>       // We are about to skip over a pair of non-identical instructions. Record
>       // if any have characteristics that would prevent reordering instructions
>       // across them.
>       BB1ItrPair.first++;
>       SkipFlagsBB1 |= skippedInstrFlags(I1);
>       if (SameLevelHoist) {
>         for (auto &SuccIterPair : OtherSuccIterPairRange) { // update flags
>           Instruction *I = &*SuccIterPair.first;
>           Instruction *I = &*SuccIterPair.first++;
>           SuccIterPair.second |= skippedInstrFlags(I);
>         }
>       }
> ```

Problem wasn't about the skipFlags. It was a small bug, I had forgotten to erase the map entry after hoisting instructions.

https://github.com/llvm/llvm-project/pull/78615