Handle OpCopy* instructions

[spencer-lunarg]

while doing #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