[Optimization] Compiler creates branches with allocs that can never be hit

[NinoFloris]

Compiler creates branches with allocs that can never be hit, or it may create superfluous allocs that are never really used, the JIT will not be able to figure that out though.

https://sharplab.io/#v2:DYLgZgzgPsCmAuACAlgO2G2iC2BDADgPL7zID2qOB+sATomSeZQLyICwAUIj1fAMYALBkwqIA7snjCuvRFEQBlMtiwAPRAFoAfEpWwAFHnw16agJSzeCgHIUsOxHdSwuXOElzBsZCPACitLgQsAAmxKQUBuaILFY8HogAnojRsYgAzPEc3Lx4AsLKqgZJ0TFQusYRzKlgAK6UGo4aANSIAEwxktI5cgpF6lq6AKzZtvZDiAAs2e4IiA0u/LAQELi0SdVRMXG5CfMpaWxZe715uAV6xaXm5ZUEW5QG9Y2TrR1dUjKn/fqITbo1GMnBNHFMgA=

let inline mapOption mapper option = 
    match option with 
    | Some x -> Some(mapper x)
    | None -> None

let almostErasedOption() =
    let y () = 3
    
    match Some(y()) |> mapOption (fun x -> x + 2) with 
    | Some x -> 5
    | None -> 4
    
let unnecessaryOption() =
    let y () = 3
    
    match Some(y()) |> mapOption (fun x -> x + 2) with 
    | Some x -> x
    | None -> 4

Decompiled:

    public static int almostErasedOption()
    {
        if (FSharpOption<int>.Some(5) == null)
        {
            return 4;
        }
        return 5;
    }

    public static int unnecessaryOption()
    {
        FSharpOption<int> fSharpOption = FSharpOption<int>.Some(5);
        if (fSharpOption == null)
        {
            return 4;
        }
        return fSharpOption.Value;
    }

The compiler is doing a pretty good job at constant folding but it really falls short by such a tiny margin to remove all the useless code.

These examples are contrived but in real cases cause inline code to not have the desired performance increase. Forcing people to write unidiomatic nullable reference returns or tricks with byref out vars.

Related:
athas/raytracers#29
dotnet/coreclr#21950 (comment)
#8953

ValueOption is worse at inlining, especially without #8970

https://sharplab.io/#v2:DYLgZgzgPsCmAuACAlgO2G2iC2BDADgPL7zID2qOB+sATomSeZQLyICwAUIj1fAMYALBkwqIA7snjCuvRFEQA1XMACusAMplsWAB6IAtAD4lK9Vp0AKPPhr1dASlm8FytbAByFLMdPuvqLBcXHBIKthkEPAAorS4ELAAJsSkFJYOiCzOPKGIAJ6I6ZmIAMzZHNy8eALCbubasJZ56RlQJjYpzIVgqpT6vvoA1IgATBmS0hVyrmaaDYj9JgCs5TP+3oYmACzlIQiIvYH8sBAQuLR5nWkZWZU5+wVFbGV3U1W4NX71Vs0Ore0EK6USw9PqbBaIYZjCRSGSvNbfPTg3SrL6eDa+LZAA

[dsyme]

I'll relabel this to feature improvement as there's specific thing that says we'll definitely remove all such bindings, especially when optimizations interact.