i965/fs: Emit ADDs for gl_FragCoord, not virtual opcodes.

i965/fs: Emit ADDs for gl_FragCoord, not virtual opcodes.

These were used only on Gen4 and 5. emit_interpolation_setup_gen6() emits
ADDs directly. The virtual opcodes weren't providing anything useful.

I'm going to repurpose these opcodes, so deleting and readding them makes
it simpler to see what's going on.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>