intel/fs/gen6: Use SEL instead of bashing thread payload for unlit centroid workaround.

intel/fs/gen6: Use SEL instead of bashing thread payload for unlit centroid workaround.

This prevents regressions on SNB due to the redundant MOVs lying
around in cases where fetch_payload_reg() returns a VGRF (currently
only in SIMD32 but soon in pretty much all cases).  The MOVs can't be
register-coalesced due to their source being a FIXED_GRF, and they
can't be copy-propagated either due to the unlit centroid workaround
partial writes.  They can be copy-propagated just fine into a SEL
instruction though.

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: 13996898 -> 14001982 (0.04%)
   instructions in affected programs: 197461 -> 202545 (2.57%)
   helped: 0
   HURT: 1251

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