i965/fs: Refactor fs_visitor::nir_setup_inputs().
i965/fs: Refactor fs_visitor::nir_setup_inputs().

No functional change.  In preparation for supporting vertex shaders,
this adds a switch statement on shader stage (since vertex attributes
and fragment shader varyings will need different handling).  It also
renames "varying" to "input", to be more general.

Signed-off-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Topi Pohjolainen <749697f19798b7f640b17221fd283b903825f071@intel.com>
Reviewed-by: Jason Ekstrand <b186559eaedf698788888c8a2559b4efb4db10d6@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler