vkd3d-shader/dxil: Handle some issues with pointers.

Forward pointers currently won't work because we don't emit an SSA value for pointers, so we can't allocate an id which will be defined later. Making VSIR support access chains is one way to resolve this. But if we do that, we should probably handle forward referenced resource handles the same way, and it would result in quite a bit of extra backend code. We probably don't need to deal with forward pointers before the next release. It would be nice to have though.

It's difficult to make dxcompiler emit forward pointer refs, so there are no tests for those.