nir: Pass through fb_fetch_output and OutputsRead from GLSL IR.

nir: Pass through fb_fetch_output and OutputsRead from GLSL IR.

The NIR representation of framebuffer fetch is the same as the GLSL
IR's until interface variables are lowered away, at which point it
will be translated to load output intrinsics.  The GLSL-to-NIR pass
just needs to copy the bits over to the NIR program.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>