i965/fs: Use the embedded compare in SEL on gen6+.

i965/fs: Use the embedded compare in SEL on gen6+.

This avoids the extra CMP and the predication on SEL, so in addition
to one less instruction, it makes scheduling less constrained.

Improves glbenchmark Egypt performance 0.6% +/- 0.2% (n=3).  Reduces
FS instruction count across affected shaders in shader-db by 1.3%
without regressing any.

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