Emit `conv.i` in pointer interop only when necessary

[ForNeVeR]

After implementing #354, we now have a feature that calls C# functions with CPtr, VoidPtr, and FuncPtr in their signatures. To call them, it will always emit a conv.i instruction.

This instruction is not always required (actually, in most cases it isn't): it should only be emitted when the value on stack is not a pointer.

We should fix that, and only emit it when necessary. See TODO[#491] when implementing.