i965: Add an option to not generate the SIMD8 fragment shader

i965: Add an option to not generate the SIMD8 fragment shader

For now, this can only be triggered with a new 'no8' INTEL_DEBUG option
and a new context flag.  We'll use the context flag later, but introducing
it now lets us bisect to this commit if it breaks something.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>