Optimisation failure with structs in referenced projects

Structs referenced from other projects have a significant optimisation failure whereby an instance of the struct cannot be passed to functions - it is always re-instantiated to empty just before the function call.

#### Repro steps

Imagine you have the following C# project with a single file:

```c#
namespace ClassLibrary1
{
public struct SomeStruct
{
private string _value;
public string Value => _value;

public void Set(string value)
{
_value = value;
}
}
}
```

The above project is referenced from F#:

```f#
open ClassLibrary1

let someFunc (s : SomeStruct) =
printfn "From inner: %A" s.Value

[]
let main argv =

let s = SomeStruct()
s.Set("there")
printfn "Pre call: %A" s.Value
someFunc s
printfn "Post call: %A" s.Value

0
```

#### Expected behavior

Output of the program is:

```
Pre call: "there"
From inner: "there"
Post call: "there"
```

#### Actual behavior

**With optimisations enabled (e.g. Release mode)**, the output is:

```
Pre call: "there"
From inner:
Post call: "there"
```

#### Known workarounds

No known workaround.

#### Related information

Severe bug, stopping me from using a C# library that uses structs.

The IL for the main function looks like:

```msil
.method public static int32 main(string[] argv) cil managed
{
.entrypoint
.custom instance void [FSharp.Core]Microsoft.FSharp.Core.EntryPointAttribute::.ctor() = ( 01 00 00 00 )
// Code size 40 (0x28)
.maxstack 4
.locals init ([0] valuetype [ClassLibrary1]ClassLibrary1.SomeStruct s,
[1] valuetype [ClassLibrary1]ClassLibrary1.SomeStruct V_1)
IL_0000: nop
IL_0001: ldloca.s V_1
IL_0003: initobj [ClassLibrary1]ClassLibrary1.SomeStruct
IL_0009: ldloc.1
IL_000a: stloc.0
IL_000b: ldloca.s s
IL_000d: ldstr "there"
IL_0012: call instance void [ClassLibrary1]ClassLibrary1.SomeStruct::Set(string)
IL_0017: ldloca.s V_1 // WHY IS THIS HERE?
IL_0019: initobj [ClassLibrary1]ClassLibrary1.SomeStruct // WHY IS THIS HERE?
IL_001f: ldloc.1
IL_0020: call void Program::someFunc(valuetype [ClassLibrary1]ClassLibrary1.SomeStruct)
IL_0025: nop
IL_0026: ldc.i4.0
IL_0027: ret
} // end of method Program::main
```

IL_0017 and IL_0019 have been added by the optimiser - in Debug mode these instructions are not here, and the bad behaviour is not present.

Thanks