nir/opt_intrinsic: optimize gl_FragDepth = gl_FragCoord.z
gl_FragDepth default value is gl_FragCoord.z so if a shader does:
gl_FragDepth = gl_FragCoord.z
we can drop this assignment.
gl_FragDepth default value is gl_FragCoord.z so if a shader does:
gl_FragDepth = gl_FragCoord.z
we can drop this assignment.