Handle OpCopy* instructions

while doing https://github.com/KhronosGroup/SPIRV-Reflect/pull/224 I realized that `OpCopy*` commands aren't correctly supported.

The following is accessing the `OpVariable` but because it goes through an `OpCopyObject` it doesn't notice it as `accessed`

```
%texture_image = OpVariable %ptr_texture UniformConstant
%var_copy = OpCopyObject %ptr_texture %texture_image
%load = OpLoad %10 %var_copy
```