i965/vs: Rework the logic for generating NIR from ARB vertex programs
i965/vs: Rework the logic for generating NIR from ARB vertex programs

Whether or not to use NIR is now equivalent to brw->scalar_vs.  We can
simplify the logic and make it far less confusing.

Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>
Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler