intel/fs/gen6: Generalize aligned_pairs_class to SIMD16 aligned barycentrics.

intel/fs/gen6: Generalize aligned_pairs_class to SIMD16 aligned barycentrics.

This is mainly meant to avoid shader-db regressions on SNB as we start
using VGRFs for barycentrics more frequently.  Currently the
aligned_pairs_class is only useful in SIMD8 mode, because in SIMD16
mode barycentric vectors are typically 4 GRFs.  This is not a problem
on Gen4-5, because on those platforms all VGRF allocations are
pair-aligned in SIMD16 mode.  However on Gen6 we end up using either
the fast or the slow path of LINTERP rather non-deterministically
based on the behavior of the register allocator.

Fix it by repurposing aligned_pairs_class to hold PLN-aligned
registers of whatever the natural size of a barycentric vector is in
the current dispatch width.

On SNB this prevents the following shader-db regressions (including
SIMD32 programs) in combination with the interpolation rework part of
this series:

   total instructions in shared programs: 13983257 -> 14527274 (3.89%)
   instructions in affected programs: 1766255 -> 2310272 (30.80%)
   helped: 0
   HURT: 11608

   LOST:   26
   GAINED: 13

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>