i965/vs: Map scalar VS input locations properly; avoid tons of MOVs.

i965/vs: Map scalar VS input locations properly; avoid tons of MOVs.

Previously, we used nir_lower_io with the scalar type_size function,
which mapped VERT_ATTRIB_* locations to...some numbers.  Then, in
fs_visitor::nir_setup_inputs(), we created temporaries indexed by
those numbers, and emitted MOVs from the actual ATTR registers to
those temporaries.  Virtually all of these were copy propagated away,
but it's still ugly.

This patch reworks our input lowering to produce NIR lower_input
intrinsics that properly index into the ATTR file, so we can access
it directly.

No changes in shader-db.

v2: Fix unreachable() message (Ken), update commit message (Matt).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>