i965: Add an INTEL_DEBUG=no16 option.

i965: Add an INTEL_DEBUG=no16 option.

Often when debugging, I don't want to see SIMD16 shaders.  It makes
INTEL_DEBUG=vs/fs output much easier to read, especially when a program
dumps many shaders.  Plus, I also want to verify that SIMD8 works before
even considering SIMD16.

v2: Fix the likeliness check (caught by Chris and Eric).

Reviewed-by: Eric Anholt <eric@anholt.net>