i965: Rewrite FS input handling to use the new NIR intrinsics.

i965: Rewrite FS input handling to use the new NIR intrinsics.

This eliminates the need to walk the list of input variables, recurse
into their types (via logic largely redundant with nir_lower_io), and
interpolate all possible inputs up front.  The backend no longer has
to care about variables at all, which eliminates complications from
trying to pack multiple variables into the same location.  Instead,
each intrinsic specifies exactly what's needed.

This should unblock Timothy's work on GL_ARB_enhanced_layouts.

Each load_interpolated_input intrinsic corresponds to PLN instructions,
while load_barycentric_at_* intrinsics correspond to pixel interpolator
messages.  The pixel/centroid/sample barycentric intrinsics simply refer
to payload fields (delta_xy[]), and don't actually generate any code.

Because we use a single intrinsic for both centroid-qualified variables
and interpolateAtCentroid(), they become indistinguishable.  We stop
sending pixel interpolator messages for those, and instead use the
payload provided data, which should be considerably faster.

On Broadwell:

total instructions in shared programs: 9067751 -> 9067570 (-0.00%)
instructions in affected programs: 145902 -> 145721 (-0.12%)
helped: 422
HURT: 209

total spills in shared programs: 2849 -> 2899 (1.76%)
spills in affected programs: 760 -> 810 (6.58%)
helped: 0
HURT: 10

total fills in shared programs: 3910 -> 3950 (1.02%)
fills in affected programs: 617 -> 657 (6.48%)
helped: 0
HURT: 10

LOST:   3
GAINED: 3

The differences mostly appear to be slight changes in MOVs.

v2: Use nir_shader_compiler_options::use_interpolated_input_intrinsics
    flag rather than passing it directly to nir_lower_io.  Use the
    unreachable() macro rather than assert in one place.  (Review
    feedback from Chris Forbes.)

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>