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

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.

Search the repository for TODO[#491] and inspect the code that emits calls for CPtr, VoidPtr, and FuncPtr signatures. Trace how the stack value's pointer status is determined, then verify that conv.i is emitted only for non-pointer values using any existing compiler tests or a focused regression test.

Written by the indexing model from the issue text.