i965: Split brw_nir_lower_inputs/outputs into per-stage functions.
i965: Split brw_nir_lower_inputs/outputs into per-stage functions.

These functions are both giant switch statements where most cases don't
overlap at all.  Let's put the bulk of the work in per-stage helpers.

Signed-off-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Iago Toral Quiroga <a847aabaac44184372f6b1cd1a59aa254daaeb0b@igalia.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler