i965/nir: Switch on shader stage in nir_lower_outputs().
i965/nir: Switch on shader stage in nir_lower_outputs().

VS, GS, and FS continue doing the same thing they did before.  We can
simplify the FS code a bit because it is always scalar.

Compute shaders now assert that there are no outputs instead of doing
a loop over 0 outputs.

Signed-off-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Jason Ekstrand <b186559eaedf698788888c8a2559b4efb4db10d6@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler