i965/nir: Pass a is_scalar boolean to brw_create_nir()

i965/nir: Pass a is_scalar boolean to brw_create_nir()

The upcoming introduction of NIR->vec4 pass will require that some NIR
lowering passes are enabled/disabled depending on the type of shader
(scalar vs. vector).

With this patch we pass a 'is_scalar' variable to the process of
constructing the NIR, to let an external context decide how the shader
should be handled.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>