intel/fs/gen7+: Implement discard/demote for SIMD32 programs.

intel/fs/gen7+: Implement discard/demote for SIMD32 programs.

At this point this simply involves fixing the initialization of the
sample mask flag register to take the right dispatch mask from the
thread payload, and fixing sample_mask_reg() to return f1.1 for the
second half of a SIMD32 thread.  This improves Manhattan 3.1
performance by 2.4%±0.31% (N>40) on my ICL with SIMD32 enabled
relative to falling back to SIMD16 for the shaders that use discard.

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