i965: Use SIMD16 instead of SIMD8 on Gen4 when possible.

i965: Use SIMD16 instead of SIMD8 on Gen4 when possible.

Gen5+ systems allow you to specify multiple shader programs - both SIMD8
and SIMD16 - and the hardware will automatically dispatch to the most
appropriate one, given the number of subspans to be processed.

However, that is not the case on Gen4.  Instead, you program a single
shader.  If you enable multiple dispatch modes (SIMD8 and SIMD16), the
shader is supposed to contain a series of jump instructions at the
beginning.  The hardware will launch the shader at a small offset,
hitting one of the jumps.

We've always thought that sounds like a pain, and weren't clear how it
affected performance - is it worth having multiple shader types?  So,
we never bothered with SIMD16 until now.

This patch takes a simpler approach: try and compile a SIMD16 shader.
If possible, set the no_8 flag, telling the hardware to just use the
SIMD16 variant all the time.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>