gl_FragDepth default value is gl_FragCoord.z so if a shader does:
gl_FragDepth = gl_FragCoord.z
we can drop this assignment.
v2: use nir_ssa_scalar_resolved and don't do this is gl_FragDepth
is wrote multiple times (Jason)
v3: - move to its own pass (Jason)
- handle var = NULL (Rhys)
v4: refactoring (Jason)
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10697>
