[PATCH] Do not transform PHI to select if doing so would be unsafe

[Rafael Ávila De Espíndola]

>> 
>> Why do you need the isa<ConstantExpr>? Can't the check be just
>> 
>>      if (!isSafeToSpeculativelyExecute(BB1V) ||
>> !isSafeToSpeculativelyExecute(BB2V))
>>        return Changed;
> 
> The reason why is for performance and correctness. I don't want to look at every instruction that can be fed into an incoming value for a phi. I just want to see the ones that can be executed *before* the phi node in the same BB as the phi. Which are these? ConstantExprs. FWIW, another part of SimplifyCFG operates in the exact same way.
> 

Got it. Thanks.
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